Send Error Object on SendError (#304)

This change ensures that when calling `requestContext.SendError` you are only able to supply parameters that match the language service beta protocol expected Error object. In other words, you have to provide an error message and optionally and error code.

# **BREAKING API CHANGES**
This will break displaying errors in Microsoft/vscode-mssql. I will be making changes to properly handle the error object shortly.

* Adding contract for returning Error objects as per LanguageService "protocol"

* Fixes throughout codebase to send only error message in error cases
Cleanup of CredentialServiceTest unit test class
Adding standard error handling for event flow validator

* Adding optional data field as per protocol spec
https://github.com/Microsoft/language-server-protocol/blob/master/protocol.md

* Adding optional validation for error objects

src/Microsoft.SqlTools.ServiceLayer/QueryExecution/Contracts/SaveResultsRequest.cs

@@ -110,17 +110,6 @@ namespace Microsoft.SqlTools.ServiceLayer.QueryExecution.Contracts
         public string Messages { get; set; }
     }
 
-    /// <summary>
-    /// Error object for save result 
-    /// </summary>
-    public class SaveResultRequestError
-    {
-        /// <summary>
-        /// Error message 
-        /// </summary>
-        public string message { get; set; }
-    }
-
     /// <summary>
     /// Request type to save results as CSV
     /// </summary>

src/Microsoft.SqlTools.ServiceLayer/QueryExecution/QueryExecutionService.cs

@@ -162,7 +162,7 @@ namespace Microsoft.SqlTools.ServiceLayer.QueryExecution
                 await requestContext.SendResult(new ExecuteRequestResult());
                 return true;
             };
-            Func<string, Task> queryCreateFailureAction = requestContext.SendError;
+            Func<string, Task> queryCreateFailureAction = message => requestContext.SendError(message);
 
             // Use the internal handler to launch the query
             return InterServiceExecuteQuery(executeParams, requestContext, queryCreateSuccessAction, queryCreateFailureAction, null, null);
@@ -228,7 +228,7 @@ namespace Microsoft.SqlTools.ServiceLayer.QueryExecution
         {
             // Setup action for success and failure
             Func<Task> successAction = () => requestContext.SendResult(new QueryDisposeResult());
-            Func<string, Task> failureAction = requestContext.SendError;
+            Func<string, Task> failureAction = message => requestContext.SendError(message);
 
             // Use the inter-service dispose functionality
             await InterServiceDisposeQuery(disposeParams.OwnerUri, successAction, failureAction);
@@ -559,10 +559,7 @@ namespace Microsoft.SqlTools.ServiceLayer.QueryExecution
             Query query;
             if (!ActiveQueries.TryGetValue(saveParams.OwnerUri, out query))
             {
-                await requestContext.SendError(new SaveResultRequestError
-                {
-                    message = SR.QueryServiceQueryInvalidOwnerUri
-                });
+                await requestContext.SendError(SR.QueryServiceQueryInvalidOwnerUri);
                 return;
             }
 
@@ -574,7 +571,7 @@ namespace Microsoft.SqlTools.ServiceLayer.QueryExecution
             ResultSet.SaveAsFailureAsyncEventHandler errorHandler = async (parameters, reason) =>
             {
                 string message = SR.QueryServiceSaveAsFail(Path.GetFileName(parameters.FilePath), reason);
-                await requestContext.SendError(new SaveResultRequestError { message = message });
+                await requestContext.SendError(message);
             };
 
             try