Clarify error message when failing to parse a pref value

Change-Id: I1b762a554913d0e3b3df0792c2ff845c4dde5a53
Reviewed-on: https://chromium-review.googlesource.com/c/1370365Reviewed-by: Dominic Battré <battre@chromium.org>
Commit-Queue: Tim Schumann <tschumann@chromium.org>
Cr-Commit-Position: refs/heads/master@{#615521}

components/sync_preferences/pref_model_associator.cc

@@ -99,8 +99,8 @@ void PrefModelAssociator::InitPrefAndAssociate(
     std::unique_ptr<base::Value> sync_value(
         reader.ReadToValue(preference.value()));
     if (!sync_value.get()) {
-      LOG(ERROR) << "Failed to deserialize preference value: "
-                 << reader.GetErrorMessage();
+      LOG(ERROR) << "Failed to deserialize value of preference '" << pref_name
+                 << "': " << reader.GetErrorMessage();
       return;
     }