Better error message when provider is not installed

[piotras]

When you set the configuration file with "midgard.configuration" and don't have the correct provider installed, you will only get an error stating "connection failed". But if you use "midgard.configuration_file", the error message says "Provider 'MySQL' not found". It would be really helpful if this error message could be shown in both cases

Copied from: midgardproject/midgard-core#193