bitbake: uievent: add error to registerEventHandler return

bitbake: uievent: add error to registerEventHandler return

Current code throws Exception("Could not register UI event handler")
if event handler can't be registered. The real reason of this is that
cooker is in busy state. Error message lacks information about this.

Added error message to the return value of registerEventHandler.
Included returned error message into the log message and exception
text.

(Bitbake rev: 07de1ca7d57dcd0cc37406feae2949da12a3fa7a)

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