linux-yocto: update validate_branches to detected invalid commit IDs

linux-yocto: update validate_branches to detected invalid commit IDs

The previous implementation of invalid commit ID checks would
error early when a bad object was detected. Rather than changing
to set +e for the entire routine, we'll capture the output and
do an explicit check for a bad object and throw a useful error
message when it is detected.

Signed-off-by: Bruce Ashfield <bruce.ashfield@windriver.com>