binman: Use an exit code when blobs are missing

At present binman returns success when told to handle missing/faked blobs
or missing bintools. This is confusing since in fact the resulting image
cannot work.

Use exit code 103 to signal this problem, with a -W option to convert
it to a warning.

Rename the flag to --ignore-missing since it controls bintools also.

Add documentation about exit codes while we are here.

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