pb-upload should display message on an HTTP error 413

[GilShalit]

Currently, when trying to load a file that is larger then what the web server (e.g. nginx) allows, the load operation is stopped with no message to the user, the name of the file is left on the component with a sand clock icon, indefinitly.

As can be seen in the console, nginx will send an error message: 413 (Request Entity Too Large).

I suggest that the component will watch out for a 413 message and will display a message like "The file being uploaded is too large. Please contact the administrator"). The file name and sand clock icon should probably be cleared.

Thanks!