Add Existence Checks to Classes & Methods in External Refinements

[rcosta358]

Added validation to ensure that classes and methods in external refinements actually exist. Also added a test for each case.
For now we'll just return an error, but in the future we'll turn these into warnings (because of dynamic generated classes).

@ExternalRefinementsFor("non.existent.Class")
public interface ErrorExtRefNonExistentClass {
    public void NonExistentClass(); 
}

Found Error: Could not find class 'non.existent.Class'

@ExternalRefinementsFor("java.util.ArrayList")
public interface ErrorExtRefNonExistentMethod<E> {

    // ...

    @StateRefinement(to = "size(this) == (size(old(this)) + 1)")
    public void adddd(E e);
}

Found Error: Could not find method 'adddd(java.lang.Object)' in class 'java.util.ArrayList'

[rcosta358]

This method does not exist in java.lang.Math.