Client: support exit-code mapping just as the builder

[epa095]

The builer maps exceptions to exit codes. It makes sense to do the same in the client, and using the same mapping.

[epa095]

Yo @milesgranger , I feel like it should be possible to make Click catch the exceptions and do the mapping for all sub-commands of the gordo command, but I did not figure out how to do it.

[milesgranger]

It seems we could inherit click.ClickException and set our desired exit_code.

class CustomException(click.ClickException):
    exit_code = 21

@click.command()
def foo():
    raise CustomException('Noob!')

Running foo will give an exit code of 21.