Uploading large files should display a notice box, and not the stack trace

When uploading large files, two types of error could be thrown. One is related to the max packet size option in mysql (see http://dev.mysql.com/doc/mysql/en/Packet_too_large.html ), and the other is related to the upload_maxsize parameter of XWiki.

These exceptions are displayed in the page as java stack traces, which is not good. We should catch these exceptions and display a clear message, describing the problem and how to fix it.