Add more detail to the 'Multiple .dist-info directories' error

pip/wheel.py

@@ -301,7 +301,9 @@ def move_wheel_files(name, req, wheeldir, user=False, home=None, root=None,
                         # is self.req.project_name case preserving?
                         s.lower().startswith(
                             req.name.replace('-', '_').lower())):
-                    assert not info_dir, 'Multiple .dist-info directories'
+                    assert not info_dir, ('Multiple .dist-info directories: ' +
+                                          destsubdir + ', ' +
+                                          ', '.join(info_dir))
                     info_dir.append(destsubdir)
             for f in files:
                 # Skip unwanted files