buildman: Detect binman reporting missing blobs

Buildman should consider a build as a success (with warnings) if missing
blobs have been dealt with by binman, even though buildman itself returns
and error code overall. This is how other warnings are dealt with.

We cannot easily access the 103 exit code, so detect the problem in the
output.

With this change, missing blobs result in an exit code of 101, although
they still indicate failure.

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