[nits] improve string description of OneSignalClientError

Motivation:
- previously it would log like this: "request failed with error: <OneSignalClientError: 0x600000cd49c0>
- now it will log like this: "request failed with error: <OneSignalClientError code: 202, message: Error parsing JSON, response: (null), underlyingError: Error Domain=NSCocoaErrorDomain Code=3840 "JSON text did not start with array or object and option to allow fragments not set. around line 1, column 0." UserInfo={NSDebugDescription=JSON text did not start with array or object and option to allow fragments not set. around line 1, column 0., NSJSONSerializationErrorIndex=0} >"

Author: nan-li

iOS_SDK/OneSignalSDK/OneSignalCore/Source/API/OneSignalClientError.m

@@ -46,4 +46,8 @@ - (instancetype)initWithCode:(NSInteger)code message:(NSString* _Nonnull)message
     return self;
 }
 
+- (NSString *)description {
+    return [NSString stringWithFormat:@"<OneSignalClientError code: %ld, message: %@, response: %@, underlyingError: %@ >", (long)_code, _message, _response, _underlyingError];
+}
+
 @end