Observe IVsHierarchy.ParseCanonicalName return value

If IVsHierarchy.ParseCanonicalName returns something other than S_OK,
then use VSITEMID.Nil

src/VisualStudio/Core/Def/Implementation/ProjectSystem/DocumentProvider.StandardTextDocument.cs

@@ -257,8 +257,9 @@ public uint GetItemId()
                 }
 
                 uint itemId;
-                Project.Hierarchy.ParseCanonicalName(_itemMoniker, out itemId);
-                return itemId;
+                return Project.Hierarchy.ParseCanonicalName(_itemMoniker, out itemId) == VSConstants.S_OK
+                    ? itemId
+                    : (uint)VSConstants.VSITEMID.Nil;
             }
         }
     }

src/VisualStudio/Core/Def/Implementation/Venus/ContainedDocument.cs

@@ -1180,8 +1180,9 @@ public uint GetItemId()
             }
 
             uint itemId;
-            Project.Hierarchy.ParseCanonicalName(_itemMoniker, out itemId);
-            return itemId;
+            return Project.Hierarchy.ParseCanonicalName(_itemMoniker, out itemId) == VSConstants.S_OK
+                ? itemId
+                : (uint)VSConstants.VSITEMID.Nil;
         }
 
         private enum RazorCodeBlockType