Tests: prevent Locking tests from killing the runner.

This takes out the global build if it throws...let's not do that.

StackExchange.Redis.Tests/Locking.cs

@@ -36,6 +36,8 @@ public void AggressiveParallel(TestMode testMode)
             using (var c2 = Create(testMode))
             {
                 void cb(object obj)
+                {
+                    try
                     {
                         var conn = (IDatabase)obj;
                         conn.Multiplexer.ErrorMessage += delegate { Interlocked.Increment(ref errorCount); };
@@ -47,6 +49,11 @@ void cb(object obj)
                         conn.Ping();
                         if (Interlocked.Decrement(ref count) == 0) evt.Set();
                     }
+                    catch (Exception ex)
+                    {
+                        Assert.True(false, "Exception in AggressiveParallel callback: " + ex.Message);
+                    }
+                }
                 int db = testMode == TestMode.Twemproxy ? 0 : 2;
                 ThreadPool.QueueUserWorkItem(cb, c1.GetDatabase(db));
                 ThreadPool.QueueUserWorkItem(cb, c2.GetDatabase(db));