Add self-hosted configuration to allow returning an exit code on error states

[jamietanna]

When Renovate runs, there are a few means to determine whether the run was successful.

One such means to identify this is i.e.

INFO: Repository finished
{
  "cloned": true,
  "durationMs": 59440,
  "result": "done",
  "status": "activated",
  "enabled": true,
  "onboarded": true
}

or an unsuccessful run:

 INFO: Repository finished (repository=local)
       "cloned": undefined,
       "durationMs": 47,
       "result": "config-validation",
       "status": "onboarded",
       "enabled": true,
       "onboarded": true

A self-hosted administrator could parse these log lines, and the exported lib/constants/error-messages.ts constants that are handily grouped, and determine whether it's a success/failure.

This is independent from being able to work out whether Renovate itself had an error (i.e. Out Of Memory errors, the process crashes for some other reason)

Alternatively, we can use the process exit code to indicate this.

This would:

• Introduce a new experimental: true self-hosted configuration option, exitCodeForErrors
• Explicitly document the known error states:
  • 1: application error
  • (...etc)
• When one of the known repository error states are encountered, the exit code is updated appropriately

Questions:

• Should this only work when one repository is set?
• Should this error code be added to Repository finished? Yes