return devtools command error through sendDevToolsMessage Promise

this enables code like:
conn.sendDevToolsMessage(...).then().catch();
which is used by lighthouse.

BUG=

Review-Url: https://codereview.chromium.org/2945763002
Cr-Commit-Position: refs/heads/master@{#480767}

headless/lib/browser/devtools_api/devtools_connection.js

@@ -31,7 +31,7 @@ class Connection {
     /**
      * An object containing pending DevTools protocol commands keyed by id.
      *
-     * @private {!Map<number, !Connection.CallbackFunction>}
+     * @private {!Map<number, !Connection.PendingCommand>}
      */
     this.pendingCommands_ = new Map();
 
@@ -42,7 +42,7 @@ class Connection {
      * An object containing DevTools protocol events we are listening for keyed
      * by name.
      *
-     * @private {!Map<string, !Map<number, !Connection.CallbackFunction>>}
+     * @private {!Map<string, !Map<number, !Connection.EventFunction>>}
      */
     this.eventListeners_ = new Map();
 
@@ -62,7 +62,7 @@ class Connection {
    *
    * @param {string} eventName Name of the DevTools protocol event to listen
    *     for.
-   * @param {!Connection.CallbackFunction} listener The callback issued when we
+   * @param {!Connection.EventFunction} listener The callback issued when we
    *     receive a DevTools protocol event corresponding to the given name.
    * @return {number} The id of this event listener.
    */
@@ -114,7 +114,7 @@ class Connection {
     this.transport_.send(
         JSON.stringify({'method': method, 'id': id, 'params': params}));
     return new Promise((resolve, reject) => {
-      this.pendingCommands_.set(id, resolve);
+      this.pendingCommands_.set(id, {resolve: resolve, reject: reject});
     });
   }
 
@@ -132,8 +132,9 @@ class Connection {
       if (!this.pendingCommands_.has(message.id))
         throw new Error('Unrecognized id:' + jsonMessage);
       if (message.hasOwnProperty('error'))
-        throw new Error('DevTools protocol error: ' + message.error);
-      this.pendingCommands_.get(message.id)(message.result);
+        this.pendingCommands_.get(message.id).reject(message.error);
+      else
+        this.pendingCommands_.get(message.id).resolve(message.result);
       this.pendingCommands_.delete(message.id);
     } else {
       if (!message.hasOwnProperty('method') ||
@@ -164,6 +165,14 @@ class Connection {
 /**
  * @typedef {function(Object): undefined|function(string): undefined}
  */
-Connection.CallbackFunction;
+Connection.EventFunction;
+
+/**
+ * @typedef {{
+ *    resolve: function(!Object),
+ *    reject: function(!Object)
+ * }}
+ */
+Connection.PendingCommand;
 
 exports = Connection;