upgrade dep:

css-loader 0.28->3.5
del 4.1->5.1
eslint 5.18->6.8
eslint-config-airbnb 17.1->18.1
eslint-loader 2.1->4.0
file-loader 2.0->6.0
fs.extra 8.0->9.0
gulp 2.0->3.0
gulp-nunjucks 4.0->5.0
json-loader 0.5->Removed
style-loader 0.23->1.1
uglifyjs-webpack-plugin 1.3->2.2
yargs 13.2->15.3
katex 0.10->0.11
react-monaco-editor 0.35->0.36
timeage-react 1.2->3.0
timeago.js 3.0->4.0
uuid 3.3->7.0

hydro/error.js

@@ -6,6 +6,14 @@ class UserFacingError extends Error {
         this.params = [];
     }
 }
+class SystemError extends Error {
+    constructor(type) {
+        super(type);
+        this.code = 500;
+        // this.stack = '';
+        this.params = [];
+    }
+}
 class BadRequestError extends UserFacingError {
     constructor(type) {
         super(type);
@@ -154,7 +162,8 @@ module.exports = {
     ValidationError, ProblemNotFoundError, TrainingNotFoundError,
     ContestNotFoundError, RecordNotFoundError, SolutionNotFoundError,
     AlreadyVotedError, ContestNotAttendedError, ContestNotLiveError,
-    ContestScoreboardHiddenError, ContestAlreadyAttendedError
+    ContestScoreboardHiddenError, ContestAlreadyAttendedError, UserFacingError,
+    SystemError
 };
 
 /*