toaster: make error message more informative

toaster: make error message more informative

Error message
 ERROR: Unprocessed MetadataEvent <bb.event.MetadataEvent object at 0x7f750e671a58>
doesn't give a lot of information about the event. It just prints
event object, which is always bb.event.MetadataEvent.

Including event type into the error message should make it more
informative:
 ERROR: Unprocessed MetadataEvent TaskArtifacts

Signed-off-by: Ed Bartosh <ed.bartosh@linux.intel.com>
Signed-off-by: Michael Wood <michael.g.wood@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>