parser: check if the last or {} block expression is valid

These checks allows for:

a) `x := call() or { 'abc' }`
b) `x := call() or { panic('abc') }`
c) `x := call() or { exit(123) }`
d) `x := call() or { continue }`
e) `x := call() or { break }`
f) `x := call() or { return }`

... but produce errors for:

g) `x := call() or { println('an error') }` , etc.

vlib/os/os_test.v

@@ -27,6 +27,7 @@ fn test_open_file() {
 	hello := 'hello world!'
 	os.open_file(filename, 'r+', 0o666) or {
 		assert err == 'No such file or directory'
+		os.File{}
 	}
 	mut file := os.open_file(filename, 'w+', 0o666) or {
 		panic(err)
@@ -212,7 +213,7 @@ fn test_is_writable_folder() {
 	tmp := os.temp_dir()
 	f := os.is_writable_folder(tmp) or {
 		eprintln('err: $err')
-		assert false
+		false
 	}
 	assert f
 }