Better error message when port is in use

NzbDrone.Host/NzbDrone.Host.csproj

@@ -125,6 +125,7 @@
     <Compile Include="Owin\NlogTextWriter.cs" />
     <Compile Include="Owin\OwinServiceProvider.cs" />
     <Compile Include="Owin\OwinTraceOutputFactory.cs" />
+    <Compile Include="Owin\PortInUseException.cs" />
     <Compile Include="PlatformValidation.cs" />
     <Compile Include="MainAppContainerBuilder.cs" />
     <Compile Include="ApplicationModes.cs" />

NzbDrone.Host/Owin/OwinHostController.cs

@@ -1,6 +1,8 @@
 using System;
 using System.Collections.Generic;
 using System.Linq;
+using System.Net;
+using System.Reflection;
 using Microsoft.Owin.Hosting;
 using NLog;
 using NzbDrone.Common.EnvironmentInfo;
@@ -50,8 +52,27 @@ namespace NzbDrone.Host.Owin
 
             _logger.Info("starting server on {0}", _urlAclAdapter.UrlAcl);
 
+            try
+            {
                 _host = WebApp.Start(OwinServiceProviderFactory.Create(), options, BuildApp);
             }
+            catch (TargetInvocationException ex)
+            {
+                if (ex.InnerException == null)
+                {
+                    throw ex;
+                }
+
+                if (ex.InnerException as HttpListenerException == null)
+                {
+                    throw ex;
+                }
+
+                throw new PortInUseException("Port {0} is already in use, please ensure NzbDrone is not already running.",
+                                             ex,
+                                             _configFileProvider.Port);
+            }
+        }
 
         private void BuildApp(IAppBuilder appBuilder)
         {

NzbDrone.Host/Owin/PortInUseException.cs

@@ -0,0 +1,15 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using NzbDrone.Common.Exceptions;
+
+namespace NzbDrone.Host.Owin
+{
+    public class PortInUseException : NzbDroneException
+    {
+        public PortInUseException(string message, Exception innerException, params object[] args) : base(message, innerException, args)
+        {
+        }
+    }
+}