hob: report event handler failures

hob: report event handler failures

If an event handler failed we were not recieving an error message -
parsing just "froze" at 99%. This is because we were expecting a
CommandFailure event and this never happened in the case of
RequestPackageInfo which is where the failure was occurring.

This also required tweaking the error formatting slightly, taking the
return value of the format function rather than the message property
since the latter only seems to contain the first line without the
traceback in the case of event handler failure. Other error cases were
tested and their message formatting is unaffected by this change.

Final part of the fix for [YOCTO #2651].

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>