main: Stringify project name in error_info

If a project can't be removed from checkout due to uncommitted changes
present, error.project is type of Project and not a string (as it is in
some cases). Project is not JSON serializable, resulting in exception
within exception handler:

TypeError: Object of type Project is not JSON serializable

This change casts project to string as a defensive mechanism. It also
passes project name instead of project object.

Change-Id: Ie7b782d73dc3647975755d5a3774d16ea6cd5348
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/413877
Tested-by: Josip Sokcevic <sokcevic@google.com>
Reviewed-by: Gavin Mak <gavinmak@google.com>
Commit-Queue: Josip Sokcevic <sokcevic@google.com>

main.py

@@ -425,7 +425,7 @@ class _Repo:
                         error_info = json.dumps(
                             {
                                 "ErrorType": type(error).__name__,
-                                "Project": project,
+                                "Project": str(project),
                                 "Message": str(error),
                             }
                         )