Send runtime error to the frontend

[greg-finley]

Closes #358

Open to other ideas as well

[nolanbconaway]

I think this approach makes sense! But is RuntimeError the correct level of specificity?

I wonder if the app should have a handler for generic errors that might not be Runtime; in which case maybe the better approach could look like a broad error handler which returns an error page that contains exception info and maybe a link to the sqlfluff issues page.

That second option seems like more code to write, so in the meantime i think we should merge in your changes. I can set up an issue and work on it this weekend if you think the error page idea makes sense.

[greg-finley]

It's up to you! I just reached for this solution to give you similar text to what the CLI would give you, which has actionable steps.

I'm actually not sure the enumeration of possible errors beyond RuntimeError but it's only one I've seen raised by lint.

I'm happy to live with this PR or let you do a different solution.

[nolanbconaway]

Ok! Lets get this in and i'll toy with an error page later on.

[nolanbconaway]

#360