Improve client error messages

Author: Alathreon

JShellAPI/src/main/java/org/togetherjava/jshellapi/exceptions/DockerException.java

@@ -1,5 +1,9 @@
 package org.togetherjava.jshellapi.exceptions;
 
+import org.springframework.http.HttpStatus;
+import org.springframework.web.bind.annotation.ResponseStatus;
+
+@ResponseStatus(value= HttpStatus.CONFLICT, reason="Session may have been forcibly destroyed.")
 public class DockerException extends Exception {
     public DockerException() {
     }

JShellAPI/src/main/resources/application.properties

@@ -16,4 +16,8 @@ jshellapi.schedulerSessionKillScanRateSeconds=60
 
 # Docker service config
 jshellapi.dockerResponseTimeout=60
-jshellapi.dockerConnectionTimeout=60
+jshellapi.dockerConnectionTimeout=60
+
+
+# So Spring includes the reason of ResponseStatusException
+server.error.include-message=always