better error handling

app/controllers/concerns/submission_scoring.rb

@@ -57,7 +57,7 @@ module SubmissionScoring
           score += output[:score] * output[:weight]
         end
 
-        if output[:status] == :timeout
+        if output.present? && output[:status] == :timeout
           output[:stderr] += "\n\n#{t('exercises.editor.timeout', permitted_execution_time: submission.exercise.execution_environment.permitted_execution_time.to_s)}"
         end
       end

lib/docker_client.rb

@@ -453,7 +453,7 @@ class DockerClient
     Timeout.timeout(@execution_environment.permitted_execution_time.to_i) do
       # TODO: check phusion doku again if we need -i -t options here
       # https://stackoverflow.com/questions/363223/how-do-i-get-both-stdout-and-stderr-to-go-to-the-terminal-and-a-log-file
-      output = container.exec(['bash', '-c',  "#{command} 1> >(tee -a /tmp/stdout.log) 2> >(tee -a /tmp/stderr.log >&2); rm /tmp/std*.log"], tty: false)
+      output = container.exec(['bash', '-c',  "#{command} 1> >(tee -a /tmp/stdout.log) 2> >(tee -a /tmp/stderr.log >&2); rm -f /tmp/std*.log"], tty: false)
     end
     Rails.logger.debug 'output from container.exec'
     Rails.logger.debug output