File Browser: Adding async task exception handling (#504)

* Replacing Dictionary with ConcurrentDictionary since values are accessed in async contexts

* Adding new method to allow async tasks to be executed in the exception continuation

* Adding unit tests for the aforementioned

* Adding exception handling to async tasks in file browser service

* Updating query execution async handling to use the async version

* Removing unnecesary send result from continuewithonfaulted

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

@@ -136,12 +136,6 @@ namespace Microsoft.SqlTools.ServiceLayer.QueryExecution
 
         #region Properties
 
-        /// <summary>
-        /// Whether the resultSet is in the process of being disposed
-        /// </summary>
-        /// <returns></returns>
-        internal bool IsBeingDisposed { get; private set; }
-
         /// <summary>
         /// The columns for this result set
         /// </summary>
@@ -506,9 +500,12 @@ namespace Microsoft.SqlTools.ServiceLayer.QueryExecution
             });
             
             // Add exception handling to the save task
-            Task taskWithHandling = saveAsTask.ContinueWithOnFaulted(t =>
+            Task taskWithHandling = saveAsTask.ContinueWithOnFaulted(async t =>
             {
-                failureHandler?.Invoke(saveParams, t.Exception.Message).Wait();
+                if (failureHandler != null)
+                {
+                    await failureHandler(saveParams, t.Exception.Message);
+                }
             });
 
             // If saving the task fails, return a failure
@@ -538,7 +535,6 @@ namespace Microsoft.SqlTools.ServiceLayer.QueryExecution
                 return;
             }
 
-            IsBeingDisposed = true;
             // Check if saveTasks are running for this ResultSet
             if (!SaveTasks.IsEmpty)
             {
@@ -550,7 +546,6 @@ namespace Microsoft.SqlTools.ServiceLayer.QueryExecution
                         fileStreamFactory.DisposeFile(outputFileName);
                     }
                     disposed = true;
-                    IsBeingDisposed = false;
                 });
             }
             else
@@ -561,14 +556,13 @@ namespace Microsoft.SqlTools.ServiceLayer.QueryExecution
                     fileStreamFactory.DisposeFile(outputFileName);
                 }
                 disposed = true;
-                IsBeingDisposed = false;
             }
         }
 
         #endregion
 
         #region Private Helper Methods
-
+        
         /// <summary>
         /// If the result set represented by this class corresponds to a single XML
         /// column that contains results of "for xml" query, set isXml = true