Add haserrormsg boolean attribute to jobset API response

This attribute allows to know if an error occurred or not: when an
error occurs, errormsg is not an empty string. Note we can not use the
errormsg attribute because it can be arbitrarily long and is excluded
from the jobset API response.

src/lib/Hydra/Controller/API.pm

@@ -87,7 +87,8 @@ sub jobsetToHash {
         checkinterval => $jobset->checkinterval,
         triggertime => $jobset->triggertime,
         fetcherrormsg => $jobset->fetcherrormsg,
-        errortime => $jobset->errortime
+        errortime => $jobset->errortime,
+        haserrormsg => $jobset->errormsg eq "" ? JSON::false : JSON::true
     };
 }