enhanced exception messages for upload errors

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | 
| License       | MIT
| Doc PR        | 

TODO
- [ ] test on Symfony app

This changes were tested on SILEX, since its what I use. If someone can test it on a symfony app, that'd be great.

It just adds more informative error messages when theres an upload error. I also changed the exceptions thrown by UploadedFile->move from "FileException" to "UploadException" - the class was there already, but never used.