Got rid of the no-longer-used (synchronous) store method.

Added a wait argument to pack.  If the wait argument is provided,
then the pack method doesn't return a value till packing is done.

Simplified the way errors are handled in normal calls and
packs by putting the error marshalling logic into a separate
method.

No longer log UndoError, VersionCommitError, and TransactionError
exceptions on the server, since these are user-level errors.

Do log pack errors.