more verbose throws in divexact

src/GroupRings.jl

@@ -432,11 +432,11 @@ end
 
 function divexact{T}(X::GroupRingElem{T}, Y::GroupRingElem{T})
    if length(Y) != 1
-      throw("Can not divide by a non-primitive element $(Y)!")
+      throw("Can not divide by a non-primitive element: $(Y)!")
    else
       idx = findfirst(Y)
       c = Y[idx]
-      c == 0 || throw("Can not invert")
+      c != 0 || throw("Can not invert: $c not found in $Y")
       g = parent(Y).basis[idx]
       return X*1//c*parent(Y)(inv(g))
    end