When presubmit checks fail, we want to print the error message from the
underlying check/tool. We use _PresubmitResult objects to collect these
diagnostics and output them to the console and json files.

This change ensures that the string values passed to the
_PresubmitResult constructor will be converted, as needed, to 'str'
instances. This way, we will avoid encoding errors and make sure to
print the actual messages instead of repr(message).

SHERIFFS: this patch breaks backwards compatibility of presubmit error
handling but we don't expect issues in downstream projects. Please
revert if downstream presubmit checks start failing spuriously.

Bug: 1225052
Change-Id: If7f5c99cb5d730c1bfbd6d108b912f77b5f2455c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3037262
Commit-Queue: Tom McKee <tommckee@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@google.com>