repo_logging: Ensure error details are printed

This updates RepoLogger.log_aggregated_errors to print out the error
message the RepoExitError when there is not a list of aggregated
errors.

Previously it would log out:
=======================================================================
Repo command failed: ManifestParseError

This told us what class of error occurred but missed the helpful error
message that developers put in the error. After this change it will now
print out the error message:

=======================================================================
Repo command failed: ManifestParseError
    error parsing manifest /path/to/manifest.xml: no element found:
    line 197, column 0

Change-Id: I4805540fddb5fa9171dbc8912becfa7fdfb1ba67
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/392614
Commit-Queue: Aravind Vasudevan <aravindvasudev@google.com>
Tested-by: Joshua Bartel <josh.bartel@garmin.com>
Reviewed-by: Aravind Vasudevan <aravindvasudev@google.com>

repo_logging.py

@@ -77,6 +77,7 @@ class RepoLogger(logging.Logger):
 
         if not err.aggregate_errors:
             self.error("Repo command failed: %s", type(err).__name__)
+            self.error("\t%s", str(err))
             return
 
         self.error(