gdb: used scoped_restore_frame in update_watchpoint

gdb: used scoped_restore_frame in update_watchpoint

I was doing some int to bool cleanup in update_watchpoint, and I
noticed a manual version of scoped_restore_selected_frame.  As always
when these things are done manually, there is the chance that, in an
error case, we might leave the wrong frame selected.

This commit updates things to use scoped_restore_selected_frame, and
also converts a local variable from int to bool.

The only user visible change after this commit is in the case where
update_watchpoint throws an error - we should now correctly restore
the previously selected frame.  Otherwise, this commit should be
invisible to the user.

Approved-By: Simon Marchi <simon.marchi@efficios.com>