cmd/exec: add --fail and --fail-defined flags (#5295)

Adds the ability to exit with a non-zero exit code for 'opa exec' by adding the
--fail and --fail-defined flags matching their respective behaviors in 'opa eval':

- Setting the --fail-defined flag allows exit of opa exec with a zero code if all
   results are undefined and there are no errors, or a non-zero code in the
   event of any defined results and/or errors.
   On non-zero exits the error message includes the number of failures/errors
   as well as a reference to the --fail-defined flag being set.
- The --fail flag behaves as the inverse of --fail-defined.

Fixes: #5007

Signed-off-by: Byron Lagrone <byron.lagrone@seqster.com>