SECOAUTH-414: metion error handling options in docs
SECOAUTH-408: deny /token requests with grant_type=implicit
SECOAUTH-403, SECOAUTH-415 Fix some error responses

I tried to make the exception responses match the spec
more accurately, but there are always some cases that
don't quite seem to fit. Feedback still welcome if
anyone finds an anomaly (and can locate the throwing
code).
Fix some links in docs