Let `finishPaintTask` in pdf_page_view.js return a promise instead, to avoid having to throw in the `paintTask.promise` rejection handler, and don't reject the `PDFPageView_draw` promise when rendering is `cancelled`

As mentioned on IRC yesterday, we currently throw even when rendering is `cancelled`, which is annoying when the devtools are active. Furthermore, since `cancelled` isn't really an error, rejecting the `PDFPageView_draw` promise seems somewhat strange in that case.

web/pdf_page_view.js

@@ -423,7 +423,7 @@ var PDFPageView = (function PDFPageViewClosure() {
 
         if (error === 'cancelled') {
           self.error = null;
-          return;
+          return Promise.resolve(undefined);
         }
 
         self.renderingState = RenderingStates.FINISHED;
@@ -459,6 +459,11 @@ var PDFPageView = (function PDFPageViewClosure() {
           pageNumber: self.id,
           cssTransform: false,
         });
+
+        if (error) {
+          return Promise.reject(error);
+        }
+        return Promise.resolve(undefined);
       };
 
       var paintTask = this.renderer === RendererType.SVG ?
@@ -468,18 +473,18 @@ var PDFPageView = (function PDFPageViewClosure() {
       this.paintTask = paintTask;
 
       var resultPromise = paintTask.promise.then(function () {
-        finishPaintTask(null);
-        if (textLayer) {
-          pdfPage.getTextContent({
-            normalizeWhitespace: true,
-          }).then(function textContentResolved(textContent) {
-            textLayer.setTextContent(textContent);
-            textLayer.render(TEXT_LAYER_RENDER_DELAY);
-          });
-        }
+        return finishPaintTask(null).then(function () {
+          if (textLayer) {
+            pdfPage.getTextContent({
+              normalizeWhitespace: true,
+            }).then(function textContentResolved(textContent) {
+              textLayer.setTextContent(textContent);
+              textLayer.render(TEXT_LAYER_RENDER_DELAY);
+            });
+          }
+        });
       }, function (reason) {
-        finishPaintTask(reason);
-        throw reason;
+        return finishPaintTask(reason);
       });
 
       if (this.annotationLayerFactory) {