Do not leave dangling promises when sending messages over protocol (#742)

It's very bad to have 'unhandled promise rejection' that can't be
handled in user code. These errors will exit node process in a near
future.

This patch avoids 'unhandled promise rejection' while sending protocol
messages.

This patch:
- introduces `puppeteer:error` debug scope and starts using it for all
  swalloed errors.
- makes sure that every `client.send` method is either awaited or its
  errors are handled.
- starts return promises from Request.continue() and Request.abort().
- starts swallow errors from Request.contine() and Request.abort().

The last is the most important part of the patch. Since
`Request.continue()` might try to continue canceled request, we should
disregard the error.

Fixes #627.

lib/helper.js

@@ -14,6 +14,7 @@
  * limitations under the License.
  */
 
+const debugError = require('debug')(`puppeteer:error`);
 /** @type {?Map<string, boolean>} */
 let apiCoverage = null;
 class Helper {
@@ -94,12 +95,11 @@ class Helper {
   static async releaseObject(client, remoteObject) {
     if (!remoteObject.objectId)
       return;
-    try {
-      await client.send('Runtime.releaseObject', {objectId: remoteObject.objectId});
-    } catch (e) {
+    await client.send('Runtime.releaseObject', {objectId: remoteObject.objectId}).catch(error => {
       // Exceptions might happen in case of a page been navigated or closed.
       // Swallow these since they are harmless and we don't leak anything in this case.
-    }
+      debugError(error);
+    });
   }
 
   /**
@@ -217,4 +217,7 @@ class Helper {
   }
 }
 
-module.exports = Helper;
+module.exports = {
+  helper: Helper,
+  debugError
+};