hob: populate error_msg when hob receives a CommandFailed event

hob: populate error_msg when hob receives a CommandFailed event

When Hob receives a bb.command.CommandFailed event, it doesn't show any error,
because the error_msg is empty. If so, it is hard to detect the issue, because
Hob continues to run till it blocks because of an information gap.

[YOCTO #5097]
Signed-off-by: Cristiana Voicu <cristiana.voicu@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>