EntityUntranslatableFieldsConstraintValidator always allows changes in default revisions

Problem/Motivation

From #2940204-24: Translatable fields with synchronization enabled should behave as untranslatable fields with respect to pending revisions:

[...] it's the logic in EntityUntranslatableFieldsConstraintValidator that's buggy, I didn't realize that earlier because field widgets are hidden in that case, but here's an example of what might happen if widgets weren't hidden:

1. Create an English article and publish it
2. Create an Italian translation and publish it
3. Save an English draft with untranslatable fields/synchronized property changes
4. Save a new Italian default revision with untranslatable fields/synchronized property changes
5. Publish the English draft -> you just overrode the Italian changes

Depending on the selected mode, you can only perform untranslatable field/sync property changes to the default revision XOR the default translation.

Proposed resolution

• Make sure changes in non-default translations are not allowed, when only the default translation is affected by untranslatable field changes, regardless of the revision being pending or default.
• Use different validation error messages for the two modes, as suggested by @Gábor Hojtsy.

Remaining tasks

• Validate the proposed solution
• Write a patch
• Reviews

User interface changes

None

API changes

None, the affected validator was not released in any stable release yet.

Data model changes

None