lang: Replace numbered errors with named ones

This makes the tests easier to read and modify without having out of
order numbers. When writing the tests, you'll remember more easily which
section you're erroring in too!

lang/interpret_test/TestAstFunc2/struct-duplicate0.output

@@ -1 +1 @@
-# err: err1: parser: `syntax error: unexpected $end, expecting EQUALS` @1:6
+# err: errLexParse: parser: `syntax error: unexpected $end, expecting EQUALS` @1:6