Possible command errors.
This is a tagged union type, introduced by fty::deftagsum.
These are structured error values that may be returned by the functions that model the commands of the wallet.
Some are directly associated to command inputs (strings) that are incorrect in some ways. Others arise when more internal processes (such as key derivations) fail. The meaning of these errors is explained in the documentation of the functions that may return them. One arises if the file that stores the wallet state has been corrupted.