Raise if we find an unknown operand

Previously, we were assuming that all unknown operands were `n`

lib/cldr/export/data/plurals/rules.rb

@@ -185,9 +185,11 @@ def to_ruby
                 # TODO: https://github.com/ruby-i18n/ruby-cldr/issues/131
                 op = "#{@type} = 0"
                 enclose = true
-              else
+              when "n"
                 fraction = true
                 op = "n.to_f"
+              else
+                raise StandardError, "Unknown plural operand `#{@type}`"
               end
               if @mod
                 op = "(" << op << ")" if enclose

test/export/data/plurals_test.rb

@@ -106,6 +106,13 @@ def test_parses_and_priority
     assert_equal([:in, [[], [3..4]], "2", "f"], [rule[1][1].operator, rule[1][1].operand, rule[1][1].mod, rule[1][1].type])
   end
 
+  def test_parse_fails_when_given_unknown_operand
+    exc = assert_raises do
+      Cldr::Export::Data::Plurals::Rule.parse("q = 0")
+    end
+    assert_equal("can not parse 'q = 0'", exc.message)
+  end
+
   def test_compiles_empty
     assert_equal(nil, Cldr::Export::Data::Plurals::Rule.parse("").to_ruby)
     assert_equal(nil, Cldr::Export::Data::Plurals::Rule.parse(" ").to_ruby)