Add syntax error for incomplete instance/class variables

### Ruby
```bash
$ ruby -e "p @"
-e:1: syntax error, unexpected $undefined, expecting $end

$ ruby -e "p @@"
-e:1: syntax error, unexpected $undefined, expecting $end
```

### mruby(current)
```bash
$ mruby -e "p @"
nil

$ mruby -e "p @@"
trace:
	[0] -e:1
NameError: uninitialized class variable "@@\377" in 8
```

### mruby(fixed)
```bash
$ mruby -e "p @"
-e:1:3: incomplete instance variable syntax

$ mruby -e "p @@"
-e:1:4: incomplete class variable syntax
```