HaikuDepot: Escape strings in the JSON builder

Fixes uploading comments which use any char that needs to be escaped.
Such as line-breaks, " or \.
HaikuDepot: Specific error for untracked packages

The web service currently only synchronizes with HaikuPorts. Packages
from 3rd party repositories which were added by the user cannot be rated.

Also, use a warning alert for errors in the UserLoginWindow and
RatePackageWindow.