Await calls with try/catch

In 451eddc647 we were a little overzealous
in removing async/await calls. Anywhere we do a try/catch around a
method that returns a Task, we need to use await.

Octokit.Tests/Helpers/NSubstituteExtensions.cs

@@ -0,0 +1,19 @@
+using System;
+using System.Threading.Tasks;
+using NSubstitute;
+using NSubstitute.Core;
+
+public static class NSubstituteExtensions
+{
+    public static ConfiguredCall ReturnsAsync<TTask, TTaskType>(
+        this TTask value, Func<CallInfo, TTaskType> returnThis,
+        params Func<CallInfo, TTaskType>[] returnThese) where TTask : Task<TTaskType>
+    {
+        return value.Returns(callInfo => Task.Factory.StartNew(() => returnThis(callInfo)));
+    }
+
+    public static ConfiguredCall ThrowsAsync<T>(this Task<T> value, Exception exception)
+    {
+        return value.ReturnsAsync<Task<T>, T>(callInfo => { throw exception; });
+    }
+}