language/python: raise if we can't detect the Python to use for venv

Library/Homebrew/language/python.rb

@@ -182,6 +182,7 @@ module Language
         if python.nil?
           pythons = %w[python python3 python@3 python@3.7 python@3.8 pypy pypy3]
           wanted = pythons.select { |py| needs_python?(py) }
+          raise FormulaUnknownPythonError, self if wanted.empty?
           raise FormulaAmbiguousPythonError, self if wanted.size > 1
 
           python = wanted.first