Error handling for expected likely subtag errors

[echeran]

Some of the test cases for likely subtags are supposed to fail. The source test data encodes this information.

We need to decide a canonical error representation during our testing. CLDR defines ways to handle such an error:

Signalling an error can be done in various ways, depending on the most consistent approach for APIs in the module. For example:

raise an exception
return an error value (such as null)
return the input (with missing fields)
return the input, but "Zzzz", and/or "ZZ" substituted for empty fields.
"und"

Whichever way we decide to handle it, we need to update our executors, which will also hopefully decrease the number of test failures.