Throw OfflineImapErrors rather than weird Exceptions

When misconfiguring OLI, e.g. by specifying a repository name that was
not configured anywhere, we would bomb out with cryptic "NoSectionError".
Throw OfflineImapError that explains what has happened. We still need to
avoid throwing exceptions with Tracebacks here though.

Signed-off-by: Sebastian Spaeth <Sebastian@SSpaeth.de>

Changelog.rst

@@ -17,6 +17,7 @@ WIP (add new stuff for the next release)
   necessary, it might have been breaking things. (J. Wiegley)
 * Improve regex that could lead to 'NoneType' object has no attribute 'group'
   (D. Franke)
+* Improved error throwing on repository misconfiguration
 
 OfflineIMAP v6.5.4 (2012-06-02)
 ===============================