Fixing -Wall warnings

[djevangelia]

There are a few -Wall warnings, mostly because of some logic branches missing return. I added return values and default cases to switches as seemed appropriate. Sometimes the code really shouldn't reach those returns and a soft bug info screen would have been useful.

I would particularly want someone familiar with the get item override system to look at get_override_search_key and get_override_search_key_by_newflag which both lack a default return, if something else should be done there.

Testing

Ran around playing and getting items for 15 minutes in Ares. I've had same changes in my R&D build for a while with no issues in Ares and Project64.

[fenhl]

See #2492 for previous discussion on this. At the very least, we should test to make sure #2486 doesn't repro on this branch.