Consider introducing a new error code for optional-only permissions

[rpl]

As a followup to #5551 we want to consider introducing a new error code to be associated to optional-only permissions being requested as non-optional.

Reasons for introducing a new error code would be:

• make it clear when a permission is invalid because non supported by Firefox vs. when it is invalid because it can only requested as optional
• point developers to a doc page describing in a bit more details how developers should use optional-only permissions (and how that differs from other optional permissions).

[Rob--W]

We have a stub for MDN documentation, which we should link that from the dedicated message: https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/optional_permissions#optional-only_permissions