[fine] Many test improvements, error improvements

- Check for more error conditions
- Move to more definitive error assertions
- Simpler error messages in some cases
- Test more conditions more thoroughly, revisit old tests

fine/tests/expression/errors/class_as_a_variable.fine

@@ -0,0 +1,8 @@
+class Foo {}
+
+fun test() -> f64 {
+  Foo + 23
+}
+
+// @expect-errors:
+// | 4:2: Foo is a class, not a value (did you mean to create a new instance with 'new'?)