mapfishapp - client: Handle error message from server when upload fails

We cannot know with current server code which is the reason - we must
display the error string created server-side, ie. not i18n to client
language.
i18n GEOR_fileupload
In case of error, reset the panel to allow to select another file
factorize code and i18n
Send 413 HTTP Status, instead of 500. Better handled on client-side too
comment
Merge branch '13.09'