Rework getValueForNumber to also cache for unknown languages

[rowanseymour]

So we don't endlessly retry loading data files that don't exist.

And we don't need to return an error if match not found because calling code checks for zero values.

cc @kuadrosx

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 72.22222% with 5 lines in your changes missing coverage. Please review.

Project coverage is 47.90%. Comparing base (b8a88b8) to head (37e0f6b).

Files with missing lines	Patch %	Lines
phonenumbers.go	72.22%	3 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #200      +/-   ##
==========================================
- Coverage   47.95%   47.90%   -0.06%     
==========================================
  Files          11       11              
  Lines        3818     3816       -2     
==========================================
- Hits         1831     1828       -3     
  Misses       1838     1838              
- Partials      149      150       +1     

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