Ignore all current dialyzer errors

The exit code from `make xref dialyzer` is now 0.

* Don't produce unmatched_returns at all
* Add all current errors to the ignore file

This is a temporary solution so that we can get the result of `make xref
dialyzer test` completely green, and get the repository running on bors.
Once this is done, this should not allow any _new_ errors into the repo.
Concurrently, we'll continue to fix dialyzer errors and _remove_ them
from the ignore file.

cc/ @Vagabond 

Here's to hoping this build is SUPERGREEN!