Fix warnings with -Wexplicit-ownership-type on fetched properties

When compiling with -Weverything (more specifically, -Wexplicit-ownership-type), clang produces the warning “method parameter of type 'NSError *__autoreleasing *' with no explicit ownership”.

__autoreleasing is the default for NSError**-like params, so this doesn’t change any behaviour, it just makes it explicit.

WDYT?