Added optional NSError parameters to set, get and remove for debugging

[Julioacarrettoni]

In order to help us find a bug I added this 3 methods so we could extract the NSError while using the keychain and log it on our system not just on NSLog as is happening right now.

• (BOOL)setObject:(nullable id)object forKey:(nonnull id)key error:(NSError * _Nullable * _Nullable) error;
• (BOOL)removeObjectForKey:(nonnull id)key error:(NSError * _Nullable * _Nullable) error;
• (nullable id)objectForKey:(nonnull id)key error:(NSError * _Nullable * _Nullable) error;

The original definition of the methods are still there to maintain backwards compatibility. So the user can still use the library as it was using it and if needed track errors.

[MrNickBarker]

I expanded on this PR in #44 to return errors also when keychain operations fail instead of just serialization errors.

[Julioacarrettoni]

I closed the PR as the original repo is no longer maintained and also because we are introducing some changes tailored to our needs I doubt other people would be interested in.