Improve auth response error handling

[jordanyordanov]

Hi @anthonyreilly

Occasionally I receive the below error when the auth response received from Salesforce is an html page instead of a json result. Possibly during Salesforce maintenance downtime. I would like to be able to customize the response deserialization and possibly extract more information from the html page in order to provide a more meaningful message instead of the below error.
I already sent you one pull request that would allow me to inject the deserialization logic and improve the error handling. Could you please kindly review it? Alternatively, if you could add exception handling that would put the html content in the exception message that would be great.

Newtonsoft.Json.JsonReaderException: Unexpected character encountered while parsing value: <. Path '', line 0, position 0.
   at Newtonsoft.Json.JsonTextReader.ParseValue()
   at Newtonsoft.Json.JsonTextReader.Read()
   at Newtonsoft.Json.JsonReader.ReadForType(JsonContract contract, Boolean hasConverter)
   at Newtonsoft.Json.Serialization.JsonSerializerInternalReader.Deserialize(JsonReader reader, Type objectType, Boolean checkAdditionalContent)
   at Newtonsoft.Json.JsonSerializer.DeserializeInternal(JsonReader reader, Type objectType)
   at Newtonsoft.Json.JsonConvert.DeserializeObject(String value, Type type, JsonSerializerSettings settings)
   at Newtonsoft.Json.JsonConvert.DeserializeObject[T](String value, JsonSerializerSettings settings)
   at NetCoreForce.Client.AuthenticationClient.<UsernamePasswordAsync>d__18.MoveNext()

[anthonyreilly]

Thanks. I'll definitely take a look. I'll look out for some instance maintenance times and see if I can capture what SFDC is returning in those cases