examples: add custom_error.v, that shows how to match over possible errors (#16265)

examples/custom_error.v

@@ -0,0 +1,20 @@
+import semver
+
+fn main() {
+	semver.from('asd') or { check_error(err) }
+	semver.from('') or { check_error(err) }
+}
+
+fn check_error(err IError) {
+	match err {
+		semver.InvalidVersionFormatError {
+			println('wrong format')
+		}
+		semver.EmptyInputError {
+			println('empty input')
+		}
+		else {
+			println('unknown error')
+		}
+	}
+}