Add MSError model

[tobihagemann]

I noticed that there isn't a way to handle errors out of the box. Basically, you have to deserialize the JSON data "manually" if the response's status code is 4xx or 5xx. I think it would simplify error handling if an MSError model was added.

Maybe it would be even useful to add something like MSErrorCode as an enum to map the code property. Even though the suggested name could be confused with MSErrorCodes of microsoftgraph/msgraph-sdk-objc.
AB#7054

[tobihagemann]

MSError.h

#import "MSObject.h"

@interface MSError : MSObject

@property (nonatomic, strong) NSString *code;
@property (nonatomic, strong) NSString *message;
@property (nonatomic, strong) MSError *innerError;

@end

MSError.m

#import "MSError.h"

@implementation MSError

- (instancetype)initWithData:(NSData *)data error:(NSError **)error {
    if (self = [super initWithData:data error:error]) {
        NSDictionary *dictionary = [self getDictionary];
        self.code = dictionary[@"code"];
        self.message = dictionary[@"message"];
        NSDictionary *innerErrorDictionary = dictionary[@"innererror"];
        if (innerErrorDictionary) {
            self.innerError = [[MSError alloc] initWithDictionary:innerErrorDictionary];
        }
    }
    return self;
}

- (instancetype)initWithDictionary:(NSDictionary *)dictionary {
    if (self = [super initWithDictionary:dictionary]) {
        self.code = dictionary[@"code"];
        self.message = dictionary[@"message"];
        NSDictionary *innerErrorDictionary = dictionary[@"innererror"];
        if (innerErrorDictionary) {
            self.innerError = [[MSError alloc] initWithDictionary:innerErrorDictionary];
        }
    }
    return self;
}

@end

[Bdar1]

I think this is a very good change and we should adopt this change.