Don't show the GUI on --verify or --upload

These are intended to be ran from the commandline, so showing the GUI
doesn't make so much sense.

This is not quite the perfect solution yet, because an Editor object and
all kinds of GUI objects are still created. This commit only prevents
them from being visible, which is a nice first step, but not quite
pretty yet. However, to do it properly, some code should be moved out of
the Editor class, so that's a bit more work.

Additionally, any messages shown with Base::showError and friends still
create a popup, they probably shouldn't do this either.
Return different exit codes with --verify or --upload

Previous commits made all failures return 1, even though originally an
unknown sketch file would return 2. This restores the previous behaviour
and adds return code 3 to mean invalid options specified.

The return codes are now:
0: Success
1: Build failed or upload failed
2: Sketch not found
3: Invalid commandline options