[backend] Create SRCSRV_SERVICE_FAIL notification if source service fails

A source service failure is similar to a build failure so bs_srcserver
should generate a notification event in this case.

Signed-off-by: Jan Blunck <jblunck@infradead.org>
[api] make debug output useful in test suite

not just showing backtrace to HoptoaddNotifier
[api] support event mails to group mail adresses

Sends mail to defined group email address if defined, otherwise to group members
as before.

There is no way atm to defined a group email adress, but to suppress mails to it.
[api] fix some of the issues in #796