Fix Warnings

I compile one of my projects with -Weverything (with some -Wno... cases) and found a bunch of warnings. I fixed warnings generated by:

-Wexplicit-ownership-type (added __autoreleasing to bunch of methods taking NSError **)
-Wcustom-atomic-properties (made some implemented properties nonatomic)
-Wreadonly-setter-attrs (omitted redundant 'strong' from some readonly properties)