Follow-up: Helpers for EPRE custom checks

[PiDelport]

This follows PR #11: I ended up hacking on this at the Codebridge community evening last night. 🙂️

This:

1. Tweaks the test assertions to check the full error content (to make sure we're actually reporting the right errors, and to have visibility of changes in output)
2. Pulls out some generic code for cell, row, and column constraint checking

(This also applies isort and black to the touched code, in the hope of standardising in that direction.)

[PiDelport]

@delenamalan: I can't add you directly as a reviewer, but you may want to have a look at this too?

[deanmalan]

Awesome! 💯 Don't you want to create a PR on the goodtables package itself as well with those Base* classes?

[PiDelport]

@delenamalan: That's an option! My instinct is to let the code settle into use here a bit first, before submitting upstream, to let rough edges in the design / interface rough be worked out. (It's a lot harder to change interface once it's an upstream public API.)

[PiDelport]

(Rebased and updated.)

[PiDelport]

@delenamalan: Awesome, thanks. :)

@jbothma: Do you want to look over this too, or happy to merge?

[jbothma]

Oh have you already brought it up to date with master? nice! I'll have a look ASAP. Thanks!