Report if uninstall of package in pep561 tests failed (#4884)

Author: emmatyping

mypy/test/testpep561.py

@@ -48,7 +48,10 @@ def install_package(self, pkg: str,
         try:
             yield
         finally:
-            run_command([python_executable, '-m', 'pip', 'uninstall', '-y', pkg], cwd=package_path)
+            returncode, lines = run_command([python_executable, '-m', 'pip', 'uninstall',
+                                             '-y', pkg], cwd=package_path)
+            if returncode != 0:
+                self.fail('\n'.join(lines))
 
     def test_get_pkg_dirs(self) -> None:
         """Check that get_package_dirs works."""