Always show full tracebacks at the end of the run

This greatly simplifies developer's life and will, possibly, allow
users familiar with Python to debug and fix the problems by
themselves.

We, possibly, should not give tracebacks for the problems like
"can't open connection", but this is up to the caller of this
routine not to provide traceback in this case.

Signed-off-by: Eygene Ryabinkin <rea@codelabs.ru>
Properly call error() function from ui.UIBase

Second argument is exception traceback, not the message; without this
tracebacks like mentioned in
  http://permalink.gmane.org/gmane.mail.imap.offlineimap.general/5712
were happening when this exception handling block was hit.

Signed-off-by: Eygene Ryabinkin <rea@codelabs.ru>
Catch two instances of untested regexp matches

They could possibly lead to the problems mentioned in
  https://github.com/OfflineIMAP/offlineimap/issues/6
though there are no sound evidences for this.

Signed-off-by: Eygene Ryabinkin <rea@codelabs.ru>