tests: add optional_method_err.vv

vlib/v/checker/tests/optional_method_err.out

@@ -0,0 +1,7 @@
+vlib/v/checker/tests/optional_method_err.vv:17:10: error: inc_to_limit() returns an option, but you missed to add an `or {}` block to it
+   15 |     mut a := Abc{}
+   16 |     for _ in 0 .. 4 {
+   17 |         _ := a.inc_to_limit(2)
+      |                ~~~~~~~~~~~~~~~
+   18 |         eprintln('a: $a')
+   19 |     }

vlib/v/checker/tests/optional_method_err.vv

@@ -0,0 +1,20 @@
+struct Abc {
+mut:
+	x int
+}
+
+fn (mut a Abc) inc_to_limit(max int) ?int {
+	if a.x >= max {
+		return error('x is already $max')
+	}
+	a.x++
+	return a.x
+}
+
+fn main() {
+	mut a := Abc{}
+	for _ in 0 .. 4 {
+		_ := a.inc_to_limit(2)
+		eprintln('a: $a')
+	}
+}