Properly re-raise exception to save original tracebacks

We usually mutate some exceptions to OfflineImapError() and it is
a whole lot better if such exception will show up with the original
traceback, so all valid occurrences of such mutations were transformed
to the 3-tuple form of "raise".  Had also added coding guidelines
document where this re-raise strategy is documented.

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

offlineimap/accounts.py

@@ -210,7 +210,8 @@ class SyncableAccount(Account):
             self._lockfd.close()
             raise OfflineImapError("Could not lock account %s. Is another "
                                    "instance using this account?" % self,
-                                   OfflineImapError.ERROR.REPO)
+                                   OfflineImapError.ERROR.REPO), \
+                  None, exc_info()[2]
 
     def _unlock(self):
         """Unlock the account, deleting the lock file"""