Show error on provider load errors to indicate the problem

This makes the error message more useful, for example when loading Unix provider without the required
Perl modules:

```
20/Mar/2014 09:56:56 [3354] error @0.011702> request to POST /login crashed: Cannot load provider Dancer::Plugin::Auth::Extensible::Provider::Unix: Can't locate Authen/Simple/PAM.pm in @INC (you may need to install the Authen::Simple::PAM module)
```