Add upper bound to `errors` dependency

[Gabriella439]

errors-2.0 will switch to use ExceptT instead of EitherT. If that will break your package then add an upper bound of errors < 2.0 to your dependencies.

[tekul]

Thanks for the advanced notice. I'll look into upgrading when I'm clear on what all the implications are. I had some issues with ExceptT/mtl in the past which made me stick with EitherT where possible.

[tekul]

Doh!