Change `-[RLMRealm writeCopyToPath:]` to follow Cocoa's error handling conventions

Interesting… I notice in `- (instancetype)initWithPath:(NSString *)path readOnly:(BOOL)readonly inMemory:(BOOL)inMemory error:(NSError **)error` that you don't check that the error instance exists before trying to assign to it.  I assumed that this would cause a crash, but maybe ARC has made this check unnecessary?