Catches OSError on IMAP connection error

When something goes wrong with the imaplib.IMAP4_SSL connection (like the host is
temporarely down or the DNS does not resolve) it generates an OSError which is currently
not catched and handled. Now OSErrors are translated to MailFetcherErrors which get
logged and the IMAP connection is retried in the next IMAP check.

Fixes the-paperless-project#474

src/documents/mail.py

@@ -216,7 +216,11 @@ def _get_messages(self):
         return r
 
     def _connect(self):
-        self._connection = imaplib.IMAP4_SSL(self._host, self._port)
+        try:
+            self._connection = imaplib.IMAP4_SSL(self._host, self._port)
+        except OSError as e:
+            msg = "Problem connecting to {}: {}".format(self._host, e.strerror)
+            raise MailFetcherError(msg)
 
     def _login(self):