Fix RunspaceHandle hang after command exception

This change fixes an issue in PowerShellContext which causes a
RunspaceHandle to not be disposed if an exception is thrown during command
execution.  A 'finally' block is now used to properly dispose of the
RunspaceHandle so that future command executions can proceed.

Author: daviwil

src/PowerShellEditorServices/Session/PowerShellContext.cs

@@ -208,6 +208,8 @@ public async Task<IEnumerable<TResult>> ExecuteCommand<TResult>(
             PSCommand psCommand,
             bool sendOutputToHost = false)
         {
+            RunspaceHandle runspaceHandle = null;
+
             // If the debugger is active and the caller isn't on the pipeline 
             // thread, send the command over to that thread to be executed.
             if (Thread.CurrentThread.ManagedThreadId != this.pipelineThreadId &&
@@ -276,7 +278,7 @@ public async Task<IEnumerable<TResult>> ExecuteCommand<TResult>(
                                 GetStringForPSCommand(psCommand)));
 
                         // Set the runspace
-                        var runspaceHandle = await this.GetRunspaceHandle();
+                        runspaceHandle = await this.GetRunspaceHandle();
                         if (runspaceHandle.Runspace.RunspaceAvailability != RunspaceAvailability.AvailableForNestedCommand)
                         {
                             this.powerShell.Runspace = runspaceHandle.Runspace;
@@ -298,6 +300,7 @@ await Task.Factory.StartNew<IEnumerable<TResult>>(
                             );
 
                         runspaceHandle.Dispose();
+                        runspaceHandle = null;
 
                         if (this.powerShell.HadErrors)
                         {
@@ -330,6 +333,13 @@ await Task.Factory.StartNew<IEnumerable<TResult>>(
                     // Write the error to the host
                     this.WriteExceptionToHost(e);
                 }
+                finally
+                {
+                    if (runspaceHandle != null)
+                    {
+                        runspaceHandle.Dispose();
+                    }
+                }
             }
 
             // TODO: Better result