UIDMaps: Better error message when not finding a mapping
UIDMaps: Better error message when not finding a mapping

Bail out with a better Exception and error text. The whole mapped
UID situation needs to be improved though.

Signed-off-by: Sebastian Spaeth <98dcb2717ddae152d5b359c6ea97e4fe34a29d4c@SSpaeth.de>

frioux/offlineimap,frioux/offlineimap