[Suggestions category] Warn more about removing categories.

components/ntp_snippets/category.cc

Index: components/ntp_snippets/category.cc
diff --git a/components/ntp_snippets/category.cc b/components/ntp_snippets/category.cc
index 7274a0105c4854aa0bb69320e9b0b2d13812a53b..0a3fc04b3be09df8e8828466b48354057fa9a200 100644
--- a/components/ntp_snippets/category.cc
+++ b/components/ntp_snippets/category.cc
@@ -22,7 +22,9 @@ Category Category::FromRemoteCategory(int remote_category) {
 
 // static
 Category Category::FromIDValue(int id) {
-  DCHECK(IsValidIDValue(id)) << "Not a valid ID: " << id;
+  DCHECK(IsValidIDValue(id)) << id << " is not a valid category ID. This may "
+                                      "have been caused by removal of a local "
+                                      "KnownCategory.";
   return Category(id);
 }