[pleroma#36] Add errors when file uploading fails

This patch adds errors when

• File is too big (intended to be merged with pleroma!523 (merged))
• General upload fail

This is my first time using Vue so I am probably doing it wrong.

TODO:

• Generic upload error and filesize checking in post_status_form
• Generic upload error and filesize checking in avatar upload
• Generic upload error and filesize checking in banner upload
• Generic upload error and filesize checking in background upload