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/repository/IMAP.py

@@ -103,7 +103,8 @@ class IMAPRepository(BaseRepository):
             except Exception as e:
                 raise OfflineImapError("remotehosteval option for repository "\
                                        "'%s' failed:\n%s" % (self, e),
-                                       OfflineImapError.ERROR.REPO)
+                                       OfflineImapError.ERROR.REPO), \
+                      None, exc_info()[2]
             if host:
                 self._host = host
                 return self._host