IMAPlib mixin class: pass 'readonly' exception to our callers

This will allow our callers who are capable of dealing with
readonly folders to properly detect this condition and act
accordingly.

One example is Gmail's "Chats" folder that is read-only,
but contains logs of the quick chats.

Tested-by: Abdó Roig-Maranges <abdo.roig@gmail.com>
Signed-off-by: Eygene Ryabinkin <rea@codelabs.ru>

offlineimap/imaplibutil.py

@@ -49,6 +49,9 @@ class UsefulIMAPMixIn(object):
         del self.untagged_responses[:]
         try:
             result = super(UsefulIMAPMixIn, self).select(mailbox, readonly)
+        except self.readonly as e:
+            # pass self.readonly to our callers
+            raise
         except self.abort as e:
             # self.abort is raised when we are supposed to retry
             errstr = "Server '%s' closed connection, error on SELECT '%s'. Ser"\