Exclusively lock Runners during code executions

Previously, the same runner could be used multiple times with different submissions simultaneously. This, however, yielded errors, for example when one submission time oud (causing the running to be deleted) while another submission was still executed.

Admin actions, such as the shell, can be still executed regardless of any other code execution.

Fixes CODEOCEAN-HG
Fixes openHPI/poseidon#423

app/errors/runner/error.rb

@@ -16,6 +16,8 @@ class Runner
 
     class Unauthorized < Error; end
 
+    class RunnerInUse < Error; end
+
     class RunnerNotFound < Error; end
 
     class FaradayError < Error; end