Improve error resilience and handling

Timeouts are now handled correctly and the Runner automatically
creates the execution environment if it could not be found in
Poseidon. The runner is deleted locally if Poseidon returns
a bad request error.

lib/runner/connection.rb

@@ -7,6 +7,7 @@ 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')))
+  TIMEOUT_EXIT_STATUS = -100
 
   def initialize(url)
     @socket = Faye::WebSocket::Client.new(url, [], ping: 5)
@@ -20,7 +21,8 @@ class Runner::Connection
     # This registers empty default callbacks.
     EVENTS.each {|event_type| instance_variable_set(:"@#{event_type}_callback", ->(e) {}) }
     @start_callback = -> {}
-    @exit_code = 0
+    # Fail if no exit status was returned.
+    @exit_code = 1
   end
 
   def on(event, &block)
@@ -80,6 +82,7 @@ class Runner::Connection
   def handle_start(_event); end
 
   def handle_timeout(_event)
+    @exit_code = TIMEOUT_EXIT_STATUS
     raise Runner::Error::ExecutionTimeout.new('Execution exceeded its time limit')
   end
 end