This commit adds the lqa_api.exit_codes module to contain all
the required exit codes used by lqa for easy access from the
API.

It initially adds the following codes: SUCCESS, OPERATION_FAILED,
and APPLICATION_ERROR.

This commit also gets the current lqa code to start using these
codes. The main change is that now lqa will exit with code 2 for
any error instead of 1, which is now used for reporting operation
failures.