[OpenACC] Implement no throw out of Compute construct

Exception handling SHOULD be possible depending on codegen (and if not,
we can make it trap and add a warning when we make that decision), but
throwing out of a compute construct is ill formed.

This patch adds an error for a 'throw' that isn't in a 'try' scope. This
error is imperfect as it won't diagnose a 'throw' that escapes its
'try', or one in a separate function, but it catches the obvious
mistakes. The other cases will need to be handled as runtime failures.