Improve helper's error reporting

[gBokiau]

Currently, when MediaHelper doesn't find an image, it returns an empty string without any further warning (even when debug is on).
When the 'media' folder is missing or wrongly configured, the Helper outputs a php notice: 'Undefined variable: dirname'

I think it would be a benefit for usability to have more helpful error reporting for these kinds of errors.

[ndm2]

I have something like that on my TODO list, I was planning to trigger notices for unresolvable paths, and throw exceptions on path misconfiguration (probably from within Config/bootstrap.php). However I'm very busy currently and I don't have time to work on this project at all... in case no one else steps in I'll give it a go in a few weeks when I have some free time again.

btw. where exactly does it return an empty string? Normally it should be either null or false in case a path couldn't be resolved. And could you supply an example (preferably in the form of a test case) that triggers the Undefined variable: dirname notice?