Improve error handling

This is a preliminary effort on #172.
A subclass of `Exception`, `ConfigurationError` is raised whenever there is an error in reading the configuration, for now that is in 2 place:
1. missing configuration file
2. site_name missing in configuration

Commi 0e66b3 address the stack trace when a user interrupts `mkdocs serve`, printing another message instead.
FYI, you don't need `pass` when you have a docstring - not a problem tho' :)