Fall back to 'edit-form' as default $rel in EntityBase::toUrl()

This is the least intrusive version possible. It still fails in the same way for entities with neither a canonical nor an edit-form link.

I think it would be slightly better to in that case throw a more helpful exception in that case and possibly we could also consider other link templates than edit-form as fallback, but I wanted to make the smallest possible change first.

As an additional improvement we could remove the override of ConfigEntityBase::toUrl() which defaults $rel to edit-form directly, but didn't do that yet, either.

This implements the additional things noted in #2.

The additional exception being thrown, while technically a behavioral change, shouldn't really affect anyone as it's the same exception class, so in effect there is not really a change, just a (slightly) more meaningful message.

The default value change in ConfigEntityBase is somewhat of a behavior change for config entities that do provide a canonical link. Those are rare, but Search API indexes, for example, provide a status overview of the index in addition to the edit-form. Search API already provides a custom delete form where the cancel link links to the canonical URL, but a similar contrib or custom module would notice a change if it were using core's default entity delete form. This is an improvement, however, as it does make sense to link to the canonical URL in that case. The only reason the override was there in the first-place was to allow the "default URL" case for config entities at all, which is still possible with this.

Good point, added a change notice.

Also fixed some stray characters in the exception message. I thought about how to test this properly and looked into actually adding a functional test for the delete form cancel URL, but our test entity types override the cancel URL of the delete form for some reason (see \Drupal\entity_test\EntityTestDeleteForm::getCancelUrl()). We do have EntityUrlTest, however, so that seemed reasonable enough to add a small test to.

Random failure

I was wondering how this could have broken BC, as I thought I/we had been careful about that, but I completely missed that Entity::toUrl() does in fact handle entities with a uri_callback and that those entities do not necessarily define any link templates.

Sorry about that! Opened #3421114: [regression] Entity::toUrl() without argument is broken for entity types with a URI callback as a (hopefully) quick follow-up to revert the unintended API change.