ChangesCollection: Return different text on multiple changes

Users are often confused when looking up a change just says "Not
found" when they are sure a change exists, but weren't aware that a
search returned multiple changes. Distinguish between these two cases
by changing the human-readable text in the response.

Change-Id: I87f6cd231be026e2bb48f26e18f11a5485aeb0e9

gerrit-server/src/main/java/com/google/gerrit/server/change/ChangesCollection.java

@@ -92,9 +92,12 @@ public class ChangesCollection implements
         }
       }
     }
-    if (ctls.size() != 1) {
+    if (ctls.isEmpty()) {
       throw new ResourceNotFoundException(id);
     }
+    if (ctls.size() != 1) {
+      throw new ResourceNotFoundException("Multiple changes found for " + id);
+    }
 
     ChangeControl ctl = ctls.get(0);
     if (!ctl.isVisible(db.get())) {