[Python] Revisit try-except blocks in the ROOT facade

There are some `except Exceptions` in the ROOT facade, which are not
much better than bare `except` because they catch any meaningful error
and turn it into an uninformative "Failed to pythonize the namespace
Foo" exception, which makes it harder to develop these namespace
pythonizations and is also not helpful to the user.

The only exception that we expect is an `ImportError` if a given ROOT
component is not available, in which case we should `pass` on the
Pythonization.

All try-except blocks are revisited accordingly:

 1. Remove `try-except` for the VecOps pythonization, because it is
    available unconditionally, as VecOps is in TMath
 2. RDF/DistRDF was already correct, I just added a comment to explain

 3. RooFit: this is the case where we had the catch-all, which is
    corrected

 4. TMVA: for some reason, the pythonization was conditional on RDF
    (probably a copy-paste error or a remnant from when the
    RBatchGenerator lived in TMVA). This is fixed and a meaningfly
    try-catch block implemented.

 5. UHI: Just like VecOps, this module is unconditionally included and
    therefore needs to `try-except`.

Author: guitargeek

bindings/pyroot/pythonizations/python/ROOT/_facade.py

@@ -327,12 +327,9 @@ def __version__(self):
     @property
     def VecOps(self):
         ns = self._fallback_getattr("VecOps")
-        try:
-            from ._pythonization._rvec import _AsRVec
+        from ._pythonization._rvec import _AsRVec
 
-            ns.AsRVec = _AsRVec
-        except Exception:
-            raise Exception("Failed to pythonize the namespace VecOps")
+        ns.AsRVec = _AsRVec
         del type(self).VecOps
         return ns
 
@@ -403,6 +400,7 @@ def MakePandasDataFrame(df):
             ns.Experimental.VariationsFor = _variationsfor(ns.Distributed.VariationsFor, ns.Experimental.VariationsFor)
             ns.Experimental.FromSpec = _fromspec(ns.Distributed.FromSpec, ns.Experimental.FromSpec)
         except ImportError:
+            # _rdf_namespace submodule not available (expected for dataframe=OFF)
             pass
 
         del type(self).RDF
@@ -422,41 +420,44 @@ def RDataFrame(self):
 
             return _rdataframe(local_rdf, ROOT._distrdf.RDataFrame)
         except ImportError:
+            # _distrdf submodule not available (expected for dataframe=OFF)
             return local_rdf
 
     # Overload RooFit namespace
     @property
     def RooFit(self):
-        from ._pythonization._roofit import pythonize_roofit_namespace
-
         ns = self._fallback_getattr("RooFit")
         try:
-            pythonize_roofit_namespace(ns)
-        except Exception:
-            raise Exception("Failed to pythonize the namespace RooFit")
+            from ._pythonization._roofit import pythonize_roofit_namespace
+        except ImportError:
+            # _roofit submodule not available (expected for roofit=OFF)
+            del type(self).RooFit
+            return ns
+
+        pythonize_roofit_namespace(ns)
+
         del type(self).RooFit
         return ns
 
     # Overload TMVA namespace
     @property
     def TMVA(self):
-        # This line is needed to import the pythonizations in _tmva directory.
-        # The comment suppresses linter errors about unused imports.
-        from ._pythonization import _tmva  # noqa: F401
-        from ._pythonization._tmva._sofie._parser._keras.parser import PyKeras
-
         ns = self._fallback_getattr("TMVA")
-        setattr(ns.Experimental.SOFIE, "PyKeras", PyKeras)
-        hasRDF = "dataframe" in self.gROOT.GetConfigFeatures()
-        if hasRDF:
-            try:
-                from ._pythonization._tmva._rtensor import _AsRTensor
-                from ._pythonization._tmva._tree_inference import SaveXGBoost
-
-                ns.Experimental.AsRTensor = _AsRTensor
-                ns.Experimental.SaveXGBoost = SaveXGBoost
-            except Exception:
-                raise Exception("Failed to pythonize the namespace TMVA")
+        try:
+            # This line is needed to import the pythonizations in _tmva directory.
+            # The comment suppresses linter errors about unused imports.
+            from ._pythonization import _tmva  # noqa: F401
+            from ._pythonization._tmva._rtensor import _AsRTensor
+            from ._pythonization._tmva._sofie._parser._keras.parser import PyKeras
+            from ._pythonization._tmva._tree_inference import SaveXGBoost
+
+            setattr(ns.Experimental.SOFIE, "PyKeras", PyKeras)
+
+            ns.Experimental.AsRTensor = _AsRTensor
+            ns.Experimental.SaveXGBoost = SaveXGBoost
+        except ImportError:
+            # _tmva submodule not available (expected for tmva=OFF)
+            pass
         del type(self).TMVA
         return ns
 
@@ -498,12 +499,9 @@ def uhi(self):
         uhi_module = types.ModuleType("uhi")
         uhi_module.__file__ = "<module ROOT>"
         uhi_module.__package__ = self
-        try:
-            from ._pythonization._uhi import _add_module_level_uhi_helpers
+        from ._pythonization._uhi import _add_module_level_uhi_helpers
 
-            _add_module_level_uhi_helpers(uhi_module)
-        except ImportError:
-            raise Exception("Failed to pythonize the namespace uhi")
+        _add_module_level_uhi_helpers(uhi_module)
         return uhi_module
 
     @property