more explicit error, e.g. for sledgehammer query operation applied in non-HOL session;
more explicit error, e.g. for sledgehammer query operation applied in non-HOL session;

MerelyAPseudonym/isabelle,MerelyAPseudonym/isabelle,MerelyAPseudonym/isabelle,MerelyAPseudonym/isabelle,MerelyAPseudonym/isabelle,MerelyAPseudonym/isabelle