feat(sign): Linked the metadata docs in add-on submission errors

[aaronanish-t]

Fixes #3636

When AMO rejects a submission with a 4xx (usually missing or invalid metadata, like the missing license in the issue), the error now ends with a hint pointing to the docs rpl linked in the issue. Server errors (5xx) are unchanged.

Example, for the error from the issue:

Submission failed (2): Bad Request
{
  "version": {
    "license": [
      "This field, or custom_license, is required for listed versions."
    ]
  }
}
The add-on metadata may be missing a required field or contain an invalid value. Pass metadata with --amo-metadata, see https://mozilla.github.io/addons-server/topics/api/addons.html#version-create and https://extensionworkshop.com/documentation/develop/getting-started-with-web-ext/#sign-and-submit-for-publication

Changes:

• fetchJson() takes an optional errorHint that's appended to 4xx errors.
• doNewAddonSubmit() and doNewAddonOrVersionSubmit() pass SUBMISSION_ERROR_HINT.
• Tests: the hint on 4xx for both submit paths, no hint on 5xx, and fetchJson() appending a hint.

tests/unit/test-util/test.submit-addon.js passes (68 tests). On Windows, the full unit suite has two failures in test.watcher.js, but they also fail intermittently on a clean master checkout, so they're unrelated.

@Rob--W, about the docs part of the issue (making it clear license is required): should that be a separate PR to the Extension Workshop repo?