Improve error handling when injecting the sidebar

If the mini-script injected into the page to determine
the content type failed to execute, the returned
'frameResults' value will be null. Unfortunately
Chrome does not expose the actual exception details.

Additionally, several Chrome async APIs were used without
checking chrome.extension.lastError in the callback.

 - Handle the case where frameResults is null or not
   an array.
 - Use utils.promisify() to wrap several Chrome async APIs.
   This checks chrome.extension.lastError and rejects the
   promise if it fails.

h/browser/chrome/lib/sidebar-injector.js

@@ -96,6 +96,14 @@ function SidebarInjector(chromeTabs, dependencies) {
     return canInject;
   }
 
+  function guessContentTypeFromURL(url) {
+    if (url.indexOf('.pdf') !== -1) {
+      return CONTENT_TYPE_PDF;
+    } else {
+      return CONTENT_TYPE_HTML;
+    }
+  }
+
   function detectTabContentType(tab) {
     if (isPDFViewerURL(tab.url)) {
       return Promise.resolve(CONTENT_TYPE_PDF);
@@ -106,16 +114,21 @@ function SidebarInjector(chromeTabs, dependencies) {
         return executeScriptFn(tab.id, {
             code: toIIFEString(detectContentType)
           }).then(function (frameResults) {
-            return frameResults[0].type;
+            if (Array.isArray(frameResults)) {
+              return frameResults[0].type;
+            } else {
+              // if the content script threw an exception,
+              // frameResults may be null or undefined.
+              //
+              // In that case, fall back to guessing based on the
+              // tab URL
+              return guessContentTypeFromURL(tab.url);
+            }
           });
       } else {
         // we cannot inject a content script in order to determine the
         // file type, so fall back to a URL-based mechanism
-        if (tab.url.indexOf('.pdf') !== -1) {
-          return Promise.resolve(CONTENT_TYPE_PDF);
-        } else {
-          return Promise.resolve(CONTENT_TYPE_HTML);
-        }
+        return Promise.resolve(guessContentTypeFromURL(tab.url));
       }
     });
   }
@@ -163,15 +176,11 @@ function SidebarInjector(chromeTabs, dependencies) {
   }
 
   function injectIntoPDF(tab) {
-    return new Promise(function (resolve, reject) {
-      if (!isPDFViewerURL(tab.url)) {
-        chromeTabs.update(tab.id, {url: getPDFViewerURL(tab.url)}, function () {
-          resolve();
-        });
-      } else {
-        resolve();
-      }
-    });
+    if (isPDFViewerURL(tab.url)) {
+      return Promise.resolve();
+    }
+    var updateFn = util.promisify(chromeTabs.update);
+    return updateFn(tab.id, {url: getPDFViewerURL(tab.url)});
   }
 
   function injectIntoLocalPDF(tab) {
@@ -252,17 +261,13 @@ function SidebarInjector(chromeTabs, dependencies) {
    *            injected script via the 'window.HYPOTHESIS_ENV' object.
    */
   function injectScript(tabId, path, env) {
-    return new Promise(function (resolve) {
-      var src  = extensionURL(path);
-
-      var code = generateMetaTagCode(env) +
-        'var script = document.createElement("script");' +
-        'script.src = "{}";' +
-        'document.body.appendChild(script);';
-      var code = code.replace('{}', src);
-
-      chromeTabs.executeScript(tabId, {code: code}, resolve);
-    });
+    var src  = extensionURL(path);
+    var code = generateMetaTagCode(env) +
+      'var script = document.createElement("script");' +
+      'script.src = "{}";' +
+      'document.body.appendChild(script);';
+    var code = code.replace('{}', src);
+    return executeScriptFn(tabId, {code: code});
   }
 }