Use ExceptT inplace of ErrorT.

[PierreR]

• Don't check file:/// protocol in optional tests
• Check all sources instead of stopping at the first failure
• Display failure position

Thanks for reviewing these changes.

I think it is alright. I had to add the Monoid instance because with ExceptT that's the new constraint (instead of Error).

There might be a way to abstract checkAllSources with a generic function but I found it more flexible to handle it explicitly.

The reason for the PR is the dependency changes (from transformer-3.x to transformers-compat).