perspectiveTransform simplify assert() for better debuggin

When using perspectiveTransform in rather complicated settings, it would be easier for developers to have two separate assertions for each part of the boolean expression in order to pinpoint problems more efficiently. In my case I am struggling in Python2.7 with finding out whether scn+1 == m.cols or the depth == CV_32F || depth == CV_64F is making a problem, which is kind of hard.
Merge pull request #2817 from bf:patch-1