refactor: harden version checks and user-agent creation based on PR feedback

Author: shuoweil

packages/google-cloud-bigquery/google/cloud/bigquery/_versions_helpers.py

@@ -264,7 +264,7 @@ def installed_version(self) -> packaging.version.Version:
                 self._installed_version = packaging.version.parse(
                     getattr(pandas_gbq, "__version__", "0.0.0")
                 )
-            except ImportError:
+            except Exception:
                 self._installed_version = packaging.version.parse("0.0.0")
 
         return self._installed_version
@@ -279,7 +279,7 @@ def delegation_api_version(self) -> int:
                 self._delegation_api_version = getattr(
                     pandas_gbq, "_internal_delegation_api_version", 0
                 )
-            except ImportError:
+            except Exception:
                 self._delegation_api_version = 0
 
         return self._delegation_api_version

packages/google-cloud-bigquery/google/cloud/bigquery/table.py

@@ -2819,7 +2819,8 @@ def to_dataframe(
                 if client_info:
                     ua = client_info.user_agent or ""
                     if "pandas-gbq" not in ua:
-                        client_info.user_agent = f"{ua} pandas-gbq/{pandas_gbq.__version__}".strip()
+                        pandas_gbq_version = getattr(pandas_gbq, "__version__", "0.0.0")
+                        client_info.user_agent = f"{ua} pandas-gbq/{pandas_gbq_version}".strip()
 
             return pandas_gbq.pandas.from_row_iterator(
                 self,