Support getOccurrencesAtPosition for 'throw' keywords.

Also revised behavior for 'return' keywords in that when the position resides on a 'return' statement, 'throw' keywords in the same function scope that are not within a try-block are also highlighted.

Fixes #768
I find this comment a little misleading... What do you mean by next owner? Really you want to convey that you are only counting "uncaught" throw statements (from the perspective of the initial Node).