Block ** and give better feedback about disabled ops

Author: drdrew42

OpenProblemLibrary/Rochester/setAlgebra11ComplexNumbers/Sqrt.pg

@@ -25,7 +25,13 @@ $showPartialCorrectAnswers = 1;
 
 Context("Complex");
 Context()->functions->disable('sqrt');
-Context()->operators->undefine('^');
+Context()->operators->undefine('^', '**');
+Context()->{error}{msg}{"Can't use '^' in this context"} 
+  = "Exponents are disabled for this problem. Compute the result for yourself and resubmit your answer.";
+Context()->{error}{msg}{"Can't use '**' in this context"} 
+  = "Exponents are disabled for this problem. Compute the result for yourself and resubmit your answer.";
+Context()->{error}{msg}{"Function 'sqrt' is not allowed in this context"} 
+  = "Square roots are disabled for this problem. Compute the result for yourself and resubmit your answer.";
 
 do {
     $z = non_zero_random(-5,5,1) + non_zero_random(-5,5,1)*i;