Introduce new error types for runners

The errors are raised in the runner model and in the runner connection
class. In the submission controller the errors are rescued and,
depending on the error, the status timeout / container depleted is
sent to the client.

lib/runner/connection.rb

@@ -4,16 +4,20 @@ require 'faye/websocket/client'
 require 'json_schemer'
 
 class Runner::Connection
+  # These are events for which callbacks can be registered.
   EVENTS = %i[start output exit stdout stderr].freeze
   BACKEND_OUTPUT_SCHEMA = JSONSchemer.schema(JSON.parse(File.read('lib/runner/backend-output.schema.json')))
 
   def initialize(url)
     @socket = Faye::WebSocket::Client.new(url, [], ping: 5)
 
+    # For every event type of faye websockets, the corresponding
+    # RunnerConnection method starting with `on_` is called.
     %i[open message error close].each do |event_type|
       @socket.on(event_type) {|event| __send__(:"on_#{event_type}", event) }
     end
 
+    # This registers empty default callbacks.
     EVENTS.each {|event_type| instance_variable_set(:"@#{event_type}_callback", ->(e) {}) }
     @start_callback = -> {}
     @exit_code = 0
@@ -43,6 +47,7 @@ class Runner::Connection
     return unless BACKEND_OUTPUT_SCHEMA.valid?(JSON.parse(event.data))
 
     event = decode(event.data)
+    # There is one `handle_` method for every message type defined in the WebSocket schema.
     __send__("handle_#{event[:type]}", event)
   end
 
@@ -50,7 +55,7 @@ class Runner::Connection
     @start_callback.call
   end
 
-  def on_error(event); end
+  def on_error(_event); end
 
   def on_close(_event)
     @exit_callback.call @exit_code
@@ -70,11 +75,11 @@ class Runner::Connection
     @output_callback.call event[:data]
   end
 
-  def handle_error(event) end
+  def handle_error(_event); end
 
-  def handle_start(event) end
+  def handle_start(_event); end
 
-  def handle_timeout(event)
-    # TODO: set the runner state
+  def handle_timeout(_event)
+    raise Runner::Error::ExecutionTimeout.new('Execution exceeded its time limit')
   end
 end