Snapshot.convert/revert: explicit error report to isolate sporadic crash;
Snapshot.convert/revert: explicit error report to isolate sporadic crash;

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