Fix different EnvDTE behaviour between SharpDevelop and Visual Studio.

Throw exception when unknown item requested from EnvDTE.ProjectItems.Item() instead of returning null.
The T4MVC template relies on this behaviour when looking for folders that do not exist.

src/AddIns/Misc/PackageManagement/Project/Src/EnvDTE/ProjectItems.cs

@@ -88,7 +88,7 @@ namespace ICSharpCode.PackageManagement.EnvDTE
 					return item;
 				}
 			}
-			return null;
+			throw new ArgumentException("Unable to find item: " + name, "name");
 		}
 		
 		internal virtual ProjectItem Item(int index)

src/AddIns/Misc/PackageManagement/Test/Src/EnvDTE/ProjectItemsTests.cs

@@ -831,5 +831,14 @@ namespace PackageManagement.Tests.EnvDTE
 			
 			Assert.AreEqual("AccountController.cs", projectItem.Name);
 		}
+		
+		[Test]
+		public void Item_UnknownProjectItemName_ThrowsException()
+		{
+			CreateProjectItems();
+			msbuildProject.AddFile("Program.cs");
+			
+			Assert.Throws<ArgumentException>(() => projectItems.Item("unknown.cs"));
+		}
 	}
 }