spec compliant 'error_description' but keep 'message' for BC

[marc-mabe]

see #958

This is only fixing one part of the problem explained in #958.
I think the other problem with missing state parameter should be done separately.

[Sephster]

Thanks @marc-mabe - although the duplication isn't ideal it is a way forwards without introducing a breaking change in the version 7 branch. Happy to merge this in.

Will look to make a full change to use error_description in version 8.

[driesvints]

This seems to have broken the Passport build but I can see that this isn't a breaking change so we should just fix our build for this: https://travis-ci.org/laravel/passport/builds/519352496

[Sephster]

Thanks @driesvints - sorry this caused you issues. We made every effort to ensure it didn't break functionality but looks like this did break your test. I will try and remember to check changes like this against Passport in future and be proactive with a fix.

[driesvints]

No worries at all. I don't consider this to be a breaking change. In the end it only broke our test suite because we're doing some a pretty aggressive assertion there. It's probably better if we used a "contains" assertion instead.