Poor user messages when `android-mode-builder` is set strangely.

[RadicalZephyr]

One of the consequences of the current structure is that when the builder variable is set to a symbol other than one of the three explicitly supported builders most android-mode commands fail with poor error messages.

I think the "best" thing to do might be to rework the way that commands are created for the specific builders or how the key-binding functions are created. Not sure which is better...

[remvee]

This is fixed in the gradle branch, right?

[RadicalZephyr]

Not really. Since it has to do with the fact that all of the android-build-* commands are built. When builder isn't one of ant, maven or gradle the case statements return nil. Which then causes the funcall to fail with a really unhelpful error.