Bug 1157201 - Prevent exception while hovering the rule-view. r=pbrosset

browser/devtools/shared/widgets/Tooltip.js

@@ -368,6 +368,8 @@ Tooltip.prototype = {
    *        A function that accepts a node argument and returns true or false
    *        (or a promise that resolves or rejects) to signify if the tooltip
    *        should be shown on that node or not.
+   *        If the promise rejects, it must reject `false` as value.
+   *        Any other value is going to be logged as unexpected error.
    *        Additionally, the function receives a second argument which is the
    *        tooltip instance itself, to be used to add/modify the content of the
    *        tooltip if needed. If omitted, the tooltip will be shown everytime.

browser/devtools/styleinspector/style-inspector-overlays.js

@@ -341,13 +341,13 @@ TooltipsOverlay.prototype = {
     let nodeInfo = this.view.getNodeInfo(target);
     if (!nodeInfo) {
       // The hovered node isn't something we care about
-      return promise.reject();
+      return promise.reject(false);
     }
 
     let type = this._getTooltipType(nodeInfo);
     if (!type) {
       // There is no tooltip type defined for the hovered node
-      return promise.reject();
+      return promise.reject(false);
     }
 
     if (this.isRuleView && this.colorPicker.tooltip.isShown()) {