New: Treat compile warnings as errors

src/Directory.Build.props

@@ -1,6 +1,8 @@
 <Project>
   <!-- Common to all Lidarr Projects -->
   <PropertyGroup>
+    <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
+
     <LidarrRootDir>$(MSBuildThisFileDirectory)..\</LidarrRootDir>
 
     <!-- Specifies the type of output -->
@@ -63,6 +65,7 @@
     
     <Deterministic Condition="$(AssemblyVersion.EndsWith('*'))">False</Deterministic>
   </PropertyGroup>
+
   <!-- Set the AssemblyConfiguration attribute for projects -->
   <ItemGroup Condition="'$(LidarrProject)'=='true'">
     <AssemblyAttribute Include="System.Reflection.AssemblyConfigurationAttribute">

src/NzbDrone.Core/Notifications/Email/EmailService.cs

@@ -51,7 +51,10 @@ public void SendEmail(EmailSettings settings, string subject, string body, bool
 
         private void Send(MailMessage email, string server, int port, bool ssl, NetworkCredential credentials)
         {
+            //TODO: This is marked obsolete in mono, will not be necessary once we build with net core instead
+#pragma warning disable CS0618
             var smtp = new SmtpClient(server, port);
+#pragma warning restore CS0618
             smtp.EnableSsl = ssl;
             smtp.Credentials = credentials;