buildman: Handle exceptions in threads gracefully

buildman: Handle exceptions in threads gracefully

There have been at least a few cases where an exception has occurred in a
thread and resulted in buildman hanging: running out of disk space and
getting a unicode error.

Handle these by collecting a list of exceptions, printing them out and
reporting failure if any are found. Add a test for this.

Signed-off-by: Simon Glass <sjg@chromium.org>