[fine] Some fixes around return statements

fine/tests/errors/return_statement_only_function.fine

@@ -0,0 +1,3 @@
+return "skidoo!";
+
+// @check-error: a return statement can only be used inside a function

fine/tests/expression/errors/return_statement_mismatch.fine

@@ -0,0 +1,8 @@
+fun test() -> f64 {
+  if false {
+    return "no way!";
+  }
+  23.0
+}
+
+// @check-error: callers of this function expect a value of type `f64` but this statement returns a value of type `string`