Provide more information on failed import.

Author: eirrgang

tests/packages/cmake_generated/src/cmake_generated/nested1/generated.py.in

@@ -1,13 +1,17 @@
 """Try to open both generated and static files from various parts of the package."""
 import sys
 from importlib.resources import files, read_text
+from types import ModuleType
 
 from .. import __version__
 
 try:
     from .. import nested2
-except ImportError:
+except ImportError as e:
     nested2 = None
+    import_error = e.msg
+else:
+    import_error = None
 
 def cmake_generated_static_data():
     return read_text("cmake_generated", "static_data").rstrip()
@@ -33,5 +37,7 @@ def cmake_generated_namespace_generated_data():
 nested_data = "success"
 
 def nested2_check():
-    if nested2 is not None:
-        return "success"
+    if import_error is not None:
+        return import_error
+    assert isinstance(nested2, ModuleType)
+    return "success"