Improve error message for 'fatal: missing job' in meta-data command

Hey @lox! This one was reported in https://github.com/buildkite/docs/issues/243 last year, and you mentioned that you wanted to improve the error message 😊

Currently (As of may last year, unless it's been updated) if you don't set the `job` flag, you get the error:

> FATAL Missing job. See: `buildkite-agent meta-data set --help`

Start by reproducing the missing-job error in the Buildkite Agent's meta-data command with the job flag omitted, then locate the command's error handling and help entry point. Review the related Buildkite docs issue for context; done means the resulting error message is more useful than the current text and the behavior is covered by the project's existing checks.

Written by the indexing model from the issue text.