When outputting JSON (i.e. --json), CLI should report errors in JSON format

[jakedoublev]

I found myself confused this morning running commands with --json because we're squashing prints to stdout (intentionally).

We should unify printing to stderr {"error":} if we have an error and the CLI should be outputting JSON, rather than nothing and just the exit code.

Acceptance Criteria

1. cli.ExitWithError should detect if the output format is JSON and structure the error output in JSON
2. cli.ExitWithWarning should detect if the output format is JSON and structure the warning output as an error in JSON
3. BATS tests go through 3-4 error/warning paths and expect JSON error output