A refusal now has a type of its own, raised where it is found: a
relative path, a folder that cannot be listed, a file that cannot be
read, has no format or cannot be converted, and an address or filter
not written as its syntax writes one. Each is still the ValueError,
LookupError or OSError read documents. Only a refusal reaches the
model as a reason, and only a refusal names a file in a folder as
failed; any other error is a fault, and is not dressed as one.

Fields that name none of a sheet's fields leave it the draft's empty
sheet, where before the reply could not be written. A file holding a
tab or line break inside a value is refused, as mtsv refuses it, and
the description and README say so. A copy that cannot be written no
longer fails the read: the map names no artifact.

Release 0.3.1 in pyproject.toml and server.json.