add specific exception class for errors when using the service locator

Blarg.GameFramework/Blarg.GameFramework.csproj

@@ -192,6 +192,7 @@
     <Compile Include="Graphics\Atlas\JsonTextureAtlasDefinition.cs" />
     <Compile Include="Graphics\Atlas\JsonTextureAtlasTile.cs" />
     <Compile Include="Graphics\Atlas\TextureAtlasLoader.cs" />
+    <Compile Include="ServiceLocatorException.cs" />
   </ItemGroup>
   <Import Project="$(MSBuildExtensionsPath32)\Microsoft\Portable\$(TargetFrameworkVersion)\Microsoft.Portable.CSharp.targets" />
   <ItemGroup>

Blarg.GameFramework/Graphics/Atlas/TextureAtlasAnimator.cs

@@ -14,7 +14,7 @@ namespace Blarg.GameFramework.Graphics.Atlas
 		{
 			_contentManager = Framework.Services.Get<ContentManager>();
 			if (_contentManager == null)
-				throw new InvalidOperationException("Could not find ContentManager object.");
+				throw new ServiceLocatorException("Could not find a ContentManager object.");
 
 			_animations = new Dictionary<string, TextureAtlasTileAnimation>();
 		}

Blarg.GameFramework/Graphics/Atlas/TextureAtlasLoader.cs

@@ -34,7 +34,7 @@ namespace Blarg.GameFramework.Graphics.Atlas
 
 			var contentManager = Framework.Services.Get<ContentManager>();
 			if (contentManager == null)
-				throw new InvalidOperationException("Cannot find a ContentManager object.");
+				throw new ServiceLocatorException("Could not find a ContentManager object.");
 
 			string textureFile = definition.Texture;
 			if (!String.IsNullOrEmpty(texturePath))

Blarg.GameFramework/Graphics/ScreenEffects/ScreenEffectManager.cs

@@ -18,7 +18,7 @@ namespace Blarg.GameFramework.Graphics.ScreenEffects
 
 			_spriteBatch = Framework.Services.Get<SpriteBatch>();
 			if (_spriteBatch == null)
-				throw new InvalidOperationException("No SpriteBatch object registered with the service locator.");
+				throw new ServiceLocatorException("Could not find a SpriteBatch object.");
 		}
 
 		#region Get / Add / Remove

Blarg.GameFramework/ServiceLocatorException.cs

@@ -0,0 +1,23 @@
+using System;
+
+namespace Blarg.GameFramework
+{
+	public class ServiceLocatorException : Exception
+	{
+		public ServiceLocatorException()
+			: base()
+		{
+		}
+
+		public ServiceLocatorException(String message)
+			: base(message)
+		{
+		}
+
+		public ServiceLocatorException(String message, Exception innerException)
+			: base(message, innerException)
+		{
+		}
+	}
+}
+