Refactor error classes

All runner errors are now in a single file. The not found error
has been splitted into an error for runner not found and for
environment not found.

app/models/runner.rb

@@ -38,7 +38,7 @@ class Runner < ApplicationRecord
   DELEGATED_STRATEGY_METHODS.each do |method|
     define_method(method) do |*args, &block|
       @strategy.send(method, *args, &block)
-    rescue Runner::Error::NotFound
+    rescue Runner::Error::RunnerNotFound
       request_new_id
       save
       @strategy.send(method, *args, &block)
@@ -55,13 +55,13 @@ class Runner < ApplicationRecord
     strategy_class = self.class.strategy_class
     self.runner_id = strategy_class.request_from_management(execution_environment)
     @strategy = strategy_class.new(runner_id, execution_environment)
-  rescue Runner::Error::NotFound
+  rescue Runner::Error::EnvironmentNotFound
     if strategy_class.sync_environment(execution_environment)
-      raise Runner::Error::NotFound.new(
+      raise Runner::Error::EnvironmentNotFound.new(
         "The execution environment with id #{execution_environment.id} was not found and was successfully synced with the runner management"
       )
     else
-      raise Runner::Error::NotFound.new(
+      raise Runner::Error::EnvironmentNotFound.new(
         "The execution environment with id #{execution_environment.id} was not found and could not be synced with the runner management"
       )
     end