Truncate error message that becomes summary_markdown build property

If a summary_markdown field longer than 4000 bytes is sent to
buildbucket's UpdateBuild API, the call is rejected.

Change-Id: I01cb5f40d98f381472a6c04292c298d7de5f9282
Reviewed-on: https://dart-review.googlesource.com/c/recipes/+/207060
Reviewed-by: Martin Kustermann <kustermann@google.com>
Commit-Queue: William Hesse <whesse@google.com>
1 file changed