[api-minor] Refactor/simplify the PDFObject class

First of all, note how there's currently *two* methods for checking if a certain object exists, which seems completely unwarranted.
Furthermore, the rarely used `getData` method (only one callsite) was renamed/rewritten to simply extend the `get` method instead. Also, there will now be a warning printed if it's used before the data is ready, which may be useful for debugging purposes.
Finally, the methods were rearranged slightly, to bring the most important ones (for an API user) to the top of the class.

src/display/annotation_layer.js

@@ -448,7 +448,7 @@ class TextWidgetAnnotationElement extends WidgetAnnotationElement {
 
       let font = null;
       if (this.data.fontRefName) {
-        font = this.page.commonObjs.getData(this.data.fontRefName);
+        font = this.page.commonObjs.getSafely(this.data.fontRefName);
       }
       this._setTextStyle(element, font);
     }

src/display/api.js

@@ -1885,7 +1885,7 @@ class WorkerTransport {
       }
 
       const [id, type, exportedData] = data;
-      if (this.commonObjs.hasData(id)) {
+      if (this.commonObjs.has(id)) {
         return;
       }
 
@@ -1937,7 +1937,7 @@ class WorkerTransport {
 
       const [id, pageIndex, type, imageData] = data;
       const pageProxy = this.pageCache[pageIndex];
-      if (pageProxy.objs.hasData(id)) {
+      if (pageProxy.objs.has(id)) {
         return;
       }
 
@@ -2257,34 +2257,31 @@ class PDFObjects {
   }
 
   /**
-   * Resolves the object `objId` with optional `data`.
+   * Returns the data of `objId` if the object exists, null otherwise.
    */
-  resolve(objId, data) {
-    const obj = this._ensureObj(objId);
-
-    obj.resolved = true;
-    obj.data = data;
-    obj.capability.resolve(data);
+  getSafely(objId) {
+    try {
+      return this.get(objId);
+    } catch (ex) {
+      warn(`PDFObjects.getSafely: "${ex && ex.message}".`);
+    }
+    return null;
   }
 
-  isResolved(objId) {
+  has(objId) {
     const obj = this._objs[objId];
     return (obj ? obj.resolved : false);
   }
 
-  hasData(objId) {
-    return this.isResolved(objId);
-  }
-
   /**
-   * Returns the data of `objId` if the object exists, null otherwise.
+   * Resolves the object `objId` with optional `data`.
    */
-  getData(objId) {
-    const obj = this._objs[objId];
-    if (!obj || !obj.resolved) {
-      return null;
-    }
-    return obj.data;
+  resolve(objId, data) {
+    const obj = this._ensureObj(objId);
+
+    obj.resolved = true;
+    obj.data = data;
+    obj.capability.resolve(data);
   }
 
   clear() {

src/display/canvas.js

@@ -810,7 +810,7 @@ var CanvasGraphics = (function CanvasGraphicsClosure() {
 
             // If the promise isn't resolved yet, add the continueCallback
             // to the promise and bail out.
-            if (!objsPool.isResolved(depObjId)) {
+            if (!objsPool.has(depObjId)) {
               objsPool.get(depObjId, continueCallback);
               return i;
             }