openapijson2schema/command.py: throw error instead of proceeding

[kevinburkesegment]

If we can't find either 'swagger' or 'openapi' keys, previously we'd get an UnboundLocalError because of an undefined variable. Even if we fixed that, it is probably bad to continue with command flow when the behavior is not well defined. Instead throw an error to exit execution immediately.

Fixes #16.

[larsks]

I'm collecting fixes in https://github.com/larsks/openapi2jsonschema. I've merged this patch into that repository.