Cancel Query Part 2 (#89)

Moving some logic around such that when a query is cancelled, it isn't thrown away, allowing reading of the partial results, a la SSMS.
Adding a configure await to fix a tenacious bug causing query cancellations to hang for 20s or more.
Capturing sql errors for user cancellation, to return all user cancelation scenarios using the same messages. (ie, cancelling during ExecuteReaderAsync will yield a error from the server whereas cancelling during ReadAsync throws a TaskCancelledException
No changes to protocol, just implementation changes.

* Test of try/finally

* Fixed issue where resultsets are unreadable after query cancellation

* Fix for await/async issue

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

@@ -201,6 +201,10 @@ namespace Microsoft.SqlTools.ServiceLayer.QueryExecution
         /// <param name="cancellationToken">Cancellation token for cancelling the query</param>
         public async Task ReadResultToEnd(CancellationToken cancellationToken)
         {
+            // Mark that result has been read
+            hasBeenRead = true;
+            fileStreamReader = fileStreamFactory.GetReader(outputFileName);
+
             // Open a writer for the file
             using (IFileStreamWriter fileWriter = fileStreamFactory.GetWriter(outputFileName, MaxCharsToStore, MaxXmlCharsToStore))
             {
@@ -221,10 +225,6 @@ namespace Microsoft.SqlTools.ServiceLayer.QueryExecution
             }
             // Check if resultset is 'for xml/json'. If it is, set isJson/isXml value in column metadata
             SingleColumnXmlJsonResultSet();
-
-            // Mark that result has been read
-            hasBeenRead = true;
-            fileStreamReader = fileStreamFactory.GetReader(outputFileName);
         }
 
         #endregion