[OC6] file upload exception handling

refs #4011 
fixes https://github.com/owncloud/mirall/issues/971

with the introduction of new more specific exceptions we have now the capability to inform the 
desktop and mobile clients (web as well for sure) about the cause of an failing upload.

Especially the desktop client can adjust its retry behavior based on these status codes.

@dragotin @danimo for review regarding desktop client
@rperezb for the mobile side
@icewind1991 @bartv2 @bantu @butonic 

THX