Use ContinueWith rather than finally for coroutine result types (micr…

…osoft#4669)

## Change
Use `ContinueWith` and `ExecuteSynchronously` to effectively make a
`finally` for the `Task` result object. Convert the existing `finally`
to only call `Complete` on exception.

src/PowerShell/CommonFiles/PowerShellCmdlet.cs

@@ -97,11 +97,14 @@ internal Task RunOnMTA(Func<Task> func)
                 this.Write(StreamType.Verbose, "Already running on MTA");
                 try
                 {
-                    return func();
+                    Task result = func();
+                    result.ContinueWith((task) => this.Complete(), TaskContinuationOptions.ExecuteSynchronously);
+                    return result;
                 }
-                finally
+                catch
                 {
                     this.Complete();
+                    throw;
                 }
             }
 
@@ -150,11 +153,14 @@ internal Task<TResult> RunOnMTA<TResult>(Func<Task<TResult>> func)
                 this.Write(StreamType.Verbose, "Already running on MTA");
                 try
                 {
-                    return func();
+                    Task<TResult> result = func();
+                    result.ContinueWith((task) => this.Complete(), TaskContinuationOptions.ExecuteSynchronously);
+                    return result;
                 }
-                finally
+                catch
                 {
                     this.Complete();
+                    throw;
                 }
             }