Use BoxError instead of Error in all places

This moves everything other than the addon code and some 'done' logic

src/apptaskmanager.js

@@ -5,6 +5,7 @@ exports = module.exports = {
 };
 
 let assert = require('assert'),
+    BoxError = require('./boxerror.js'),
     debug = require('debug')('box:apptaskmanager'),
     fs = require('fs'),
     locker = require('./locker.js'),
@@ -42,7 +43,7 @@ function scheduleTask(appId, taskId, callback) {
     if (!gInitialized) initializeSync();
 
     if (appId in gActiveTasks) {
-        return callback(new Error(`Task for %s is already active: ${appId}`));
+        return callback(new BoxError(BoxError.CONFLICT, `Task for %s is already active: ${appId}`));
     }
 
     if (Object.keys(gActiveTasks).length >= TASK_CONCURRENCY) {