Fixed status 500 when creating player

gaming-rest/src/main/java/cz/muni/fi/pa165/rest/exception/ResourceAlreadyExistsException.java

 package cz.muni.fi.pa165.rest.exception;
 
+import org.springframework.http.HttpStatus;
+import org.springframework.web.bind.annotation.ResponseStatus;
+
+@ResponseStatus(value = HttpStatus.UNPROCESSABLE_ENTITY, reason="Player with the specified uuid already exists")
 public class ResourceAlreadyExistsException extends RuntimeException{
+
 }