Validation error reporting functionality.

Changed a few things so we can report errors to users properly in the
config loading system.

 - We now return from read_mediagoblin_config both a loaded config and
   the validation results
 - We now have a helper function generate_validation_report that can
   generate a proper validation report saying if there are errors in a
   way that's useful to users.
 - Moved conf->config in the read_mediagoblin_config function, which
   looks nicer IMO.