Fixing issue #602

Now if you use ```!N:Nat``` in the rule, the kompiler will catch this error and give the following error message:
```
[Error] Compiler: Unsupported sort of fresh variable: Nat
Only sorts [Id, Int] admit fresh variables.
	File: ...
	Location: (24,20,24,26)
	Compilation Phase: class org.kframework.compile.checks.CheckVariables
```

@andreistefanescu please review