Tweak the goto file/line command (in the right button menu in PyShell

Tweak the goto file/line command (in the right button menu in PyShell
and output windows) so that it if it doesn't succeed with the line
at the cursor, it tries the line before that.  This is handy with
tracebacks, where my natural tendency is to click in the displayed
source line rather than in the file/line indicator just above it.
Now I can indulge this tendency.

I factored out a helper and changed the error handling so that a
non-existing file is treated as if the line didn't match -- this is
handy because some function calls (e.g. "foo.bar(1)") match the grep
pattern.