Updating the error message for unbound variables. Fixing issue #684.

@dwightguth @grosu please review.
Now the error message will say: 
```
[Error] Compiler: Unbounded variable X:KList should start with ? or ! to make
it fresh.
	File: D:\work\k\samples\test\test.k
	Location: (6,17,6,18)
	Compilation Phase: class org.kframework.compile.checks.CheckVariables
```
I would get rid of "to make it fresh".  Other than that, it is exactly what we wanted here.  Thanks, Radu.