It is easy to get type confusion and end up passing a list as the
message parameter to _PresubmitResult. This error will not be detected
until the end of the run - perhaps hours later - when all evidence of
where the list came from is lost.

This change ensures that the message parameter is a string. If it is not
then the exception that is thrown should allow quick identification of
the problematic code.

This also fixes a presubmit unit test that passed None as the message.
We could support that but I don't think that we should.

Bug: 1309977
Change-Id: Ifb1d5100d47922b0ebd8bb834caa6fbba690b43c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3566436Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Commit-Queue: Bruce Dawson <brucedawson@chromium.org>