Clarify EnvironmentNotFound error handling

app/models/runner.rb

@@ -78,13 +78,17 @@ class Runner < ApplicationRecord
       self.runner_id = strategy_class.request_from_management(execution_environment)
       @strategy = strategy_class.new(runner_id, execution_environment)
     rescue Runner::Error::EnvironmentNotFound
+      # Whenever the environment could not be found by the runner management, we
+      # try to synchronize it and then forward a more specific error to our callee.
       if strategy_class.sync_environment(execution_environment)
         raise Runner::Error::EnvironmentNotFound.new(
-          "The execution environment with id #{execution_environment.id} was not found and was successfully synced with the runner management"
+          "The execution environment with id #{execution_environment.id} was not found yet by the runner management. "\
+          'It has been successfully synced now so that the next request should be successful.'
         )
       else
         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"
+          "The execution environment with id #{execution_environment.id} was not found by the runner management."\
+          'In addition, it could not be synced so that this probably indicates a permanent error.'
         )
       end
     end