Fixed unit tests for studygroup/user specs

I was thinking it could be redundant to check since once we check the user isn't even in the studygroup s/he wants to leave, we would throw that error first so the second check may never get used. 
For the case where a student was trying to leave a studygroup that s/he wasn't already in AND isn't even enrolled in the class associated with it, it would be most helpful/make sense to display that the student wasn't even a member of the studygroup to begin. 

But we can add it to be safe