Variables: Import Data Sets reports why a cell was refused

dataset.import replaced the reason for a refused cell with "value for
`X` must match its variable kind", so a visibility value such as
maybe and a graph value over 262,144 bytes gave the same message. The
error now keeps the line and gives the reason, and both messages have
a row in every complete-message catalog.

docs/mcp.md

@@ -1644,7 +1644,8 @@ already walks Image Trace. Short agent entry points:
   `dataset.export {path?}` (CSV: `name`, then one column per variable). `dataset.import` also reads tab-separated
   files, and files whose first header cell isn't `name`: every header cell is then a variable, and the rows take the
   first free names `Data Set 1`, `Data Set 2` and so on. An import error about a row's values or name gives the file
-  line the row starts on. A header cell that names no variable defines one, unbound, in the same undo step as the
+  line the row starts on, and an error about a value says why it was refused: a visibility value other than true,
+  false, 1, 0, yes or no, or a graph value longer than 262,144 bytes. A header cell that names no variable defines one, unbound, in the same undo step as the
   rows. `#Shown` defines a visibility variable `Shown`, `@Photo` defines an image variable `Photo`, and a cell without
   a prefix defines a text variable. A prefixed cell whose name is already a variable of the prefix's kind names it;
   one whose name is a variable of another kind fails the import. A `%` cell that isn't a variable's exact name (graph