Throw exception if configuration file has unexpected extension

Hi,

We recently had an issue where our Logback XML configuration file (specified via a system property) didn't have the extension ".xml". This didn't work, but we didn't get any feedback on it (other than no entries in our log file).

This change (with included unit tests) makes logback-classic throw an exception when provided with a file without the appropriate extension.

Sincerely,
Halvard