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-duplicate1.output

@@ -1 +1 @@
-# err: err1: can't set return type in parser: `can't set return type in parser: duplicate struct field of `x int`` @1:24
+# err: errLexParse: can't set return type in parser: `can't set return type in parser: duplicate struct field of `x int`` @1:24