Add OfflineImapError class

This Exception can be thrown whenever a sync error occurs. It is used
for outputting sensible error messages to the user and it denotes a
"severity", it can tell offlineimap if we need to abort a message, a
folder, a repo sync, or whether we should indeed abort immediately.

The exception is not yet used in the code.

Signed-off-by: Sebastian Spaeth <Sebastian@SSpaeth.de>
Signed-off-by: Nicolas Sebrecht <nicolas.s-dev@laposte.net>

offlineimap/__init__.py

@@ -18,6 +18,7 @@ to distribute it under the conditions laid out in COPYING."""
 __homepage__ = "http://github.com/nicolas33/offlineimap"
 __license__  = "Licensed under the GNU GPL v2+ (v2 or any later version)."
 
+from offlineimap.error import OfflineImapError
 # put this last, so we don't run into circular dependencies using 
 # e.g. offlineimap.__version__.
 from offlineimap.init import OfflineImap