Add Semmle LGTM code review

Also fix two issues detected by LGTM.

README.md

@@ -4,6 +4,7 @@ Project planning for YouTrack. Reconstruct past project schedule from YouTrack a
 
 [![Build Status](https://travis-ci.org/fschopp/project-planning-for-you-track.svg?branch=master)](https://travis-ci.org/fschopp/project-planning-for-you-track)
 [![Coverage Status](https://coveralls.io/repos/github/fschopp/project-planning-for-you-track/badge.svg?branch=master)](https://coveralls.io/github/fschopp/project-planning-for-you-track?branch=master)
+[![Language grade: JavaScript](https://img.shields.io/lgtm/grade/javascript/g/fschopp/project-planning-for-you-track.svg?logo=lgtm&logoWidth=18)](https://lgtm.com/projects/g/fschopp/project-planning-for-you-track/context:javascript)
 [![npm](https://img.shields.io/npm/v/@fschopp/project-planning-for-you-track.svg)](https://www.npmjs.com/package/@fschopp/project-planning-for-you-track)
 
 ## Overview

src/main/you-track-project-planning.ts

@@ -1,5 +1,5 @@
 import { strict as assert } from 'assert';
-import { Failure, ProgressCallback, ProjectPlan, YouTrackConfig, YouTrackIssue, } from './api-types';
+import { ProgressCallback, ProjectPlan, YouTrackConfig, YouTrackIssue, } from './api-types';
 import { OnlyOptionals } from './util';
 import { httpGet, httpGetAll, httpGetAllWithOptions } from './you-track-http';
 import * as RestApi from './you-track-rest';

src/mocks/you-track-database.ts

@@ -540,7 +540,7 @@ export default class YouTrackDatabase {
         } as DeepPartial<CustomField>, 200];
       case CUSTOM_FIELD_ID_REMAINING_EFFORT: case CUSTOM_FIELD_ID_REMAINING_WAIT: case CUSTOM_FIELD_ID_ASSIGNEE:
       case CUSTOM_FIELD_ID_TYPE:
-        throw Error('REST endpoint for getting custom field with id ${id} not implemented.');
+        throw Error(`REST endpoint for getting custom field with id ${id} not implemented.`);
       default: return [youTrackError('Not Found', `Entity with id ${id} not found`), 404];
     }
   }