chore: make the lazy import error message clearer (#9667)

* made the lazy import error message clearer

* add header

Author: SaraCalla

haystack/lazy_imports.py

@@ -46,8 +46,8 @@ def __exit__(
         """
         if isinstance(exc_value, ImportError):
             message = (
-                f"Failed to import '{exc_value.name}'. {self.import_error_msg.format(exc_value.name)}. "
-                f"Original error: {exc_value}"
+                f"Haystack failed to import the optional dependency '{exc_value.name}'. "
+                f"{self.import_error_msg.format(exc_value.name)}. Original error: {exc_value}"
             )
             self._deferred = (exc_value, message)
             return True

releasenotes/notes/improve-lazy-import-err-msg-cc603e5ff5ac8b0e.yaml

@@ -0,0 +1,4 @@
+---
+enhancements:
+  - |
+    Made the lazy import error message clearer explaining that the optional dependency is missing.

test/test_lazy_imports.py

@@ -0,0 +1,22 @@
+# SPDX-FileCopyrightText: 2022-present deepset GmbH <info@deepset.ai>
+#
+# SPDX-License-Identifier: Apache-2.0
+
+import pytest
+
+from haystack.lazy_imports import DEFAULT_IMPORT_ERROR_MSG, LazyImport
+
+
+class TestLazyImport:
+    def test_import_error_is_suppressed_and_deferred(self):
+        with LazyImport() as lazy_import:
+            import a_module
+
+        assert lazy_import._deferred is not None
+        exc_value, message = lazy_import._deferred
+        assert isinstance(exc_value, ImportError)
+        expected_message = (
+            "Haystack failed to import the optional dependency 'a_module'. Try 'pip install a_module'. "
+            "Original error: No module named 'a_module'"
+        )
+        assert expected_message in message