Improve auth flow error handling

[tkleczek]

Overview:
Fixes several unrelated problems related to auth flow error handling

What problem does it solve?:

1. Fixes a case of wrong error returned in case of unsupported grant type (according to RFC should be "unsupported_grant_type", not "invalid_grant").
2. Fixes "connector not found" error displayed in an inconsistent not-end-user-friendly way (errTokenHelper used instead of renderError)
3. Makes errors related to invalid connector in parseAuthorizationRequest redirect back to the client (as RFC states that after client_id/redirect uri is validated, all further errors should be redirected to client)
4. Makes two type of errors returned from parseAuthorizationRequest explicit (either redirected error or displayed error)

This PR partially fixes #1796

Special notes for a reviewer:

[sagikazarmark]

@tkleczek can you please rebase your PR?

[tkleczek]

@sagikazarmark done, sorry for the delay

[nabokihms]

I left some comments, but overall it looks good! Thanks!

[sagikazarmark]

@tkleczek I know this has been in the queue for a long time, but I'd like to release it. Do you think you can take a look at @nabokihms comments?

[sagikazarmark]

Moving to the next milestone

[tkleczek]

@sagikazarmark @nabokihms I rebased the PR and fixed code review issues.

[nabokihms]

Everything looks good, thanks. I added some NIT comments.

I am going to test this PR. If everything is ok, I will mark it as approved.

[nabokihms]

The only problem I found is that we still see a displayed error on accessing the /auth endpoint with the wrong connector id. This happens because now we are parsing an auth request only after redirecting to the connector login page.

I don't think this blocks us from accepting this PR as is. Let's merge it. We can deal with the rest later.

[nabokihms]

@tkleczek thank you for the contribution!