accounts.py: Print the stacktrace via traceback module

All other instances were converted to format crash output including a
stacktrace, but this one seems to have been left out. Make Exceptions
print their stacktrace here too.

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

offlineimap/accounts.py

@@ -379,4 +379,5 @@ def syncfolder(accountname, remoterepos, remotefolder, localrepos,
         raise
     except:
         ui.warn("ERROR in syncfolder for %s folder %s: %s" % \
-                (accountname,remotefolder.getvisiblename(),sys.exc_info()[1]))
+                (accountname,remotefolder.getvisiblename(),
+                 traceback.format_exc()))