Fixed Error Swallowing outlined in issue 5354

Author: Vikram - Google

src/pip/_vendor/__init__.py

@@ -32,15 +32,11 @@ def vendored(modulename):
     try:
         __import__(modulename, globals(), locals(), level=0)
     except ImportError:
-        # We can just silently allow import failures to pass here. If we
-        # got to this point it means that ``import pip._vendor.whatever``
-        # failed and so did ``import whatever``. Since we're importing this
-        # upfront in an attempt to alias imports, not erroring here will
-        # just mean we get a regular import error whenever pip *actually*
-        # tries to import one of these modules to use it, which actually
-        # gives us a better error message than we would have otherwise
-        # gotten.
-        pass
+        # This error used to be silenced in earlier variants of this file, to instead
+        # raise the error when pip actually tries to use the missing module. 
+        # Based on inputs in #5354, this was changed to explicitly raise the error.
+        # Re-raising the exception without modifying it is an intentional choice. 
+        raise
     else:
         sys.modules[vendored_name] = sys.modules[modulename]
         base, head = vendored_name.rsplit(".", 1)