docs(config): scope the configuration-error claim to what is actually reported

Only the config checks name the variable; a syntax error or an unknown
column fails later in the view build and the abort names the table.

docs/derived-variables.md

@@ -76,7 +76,7 @@ Derived columns themselves are an EDIT screen feature: the VIEW screen and the d
  - **Read-only.** Derived columns cannot be edited in the grid, and any value submitted for them is discarded before the approval package is created.
  - **Recomputed on every read.** The value always reflects the current data and the current definition, and there is no history of previous derived values.
 
-A configuration error - a bad expression, an unknown column, a name collision - stops the EDIT screen with a message naming the variable, rather than silently dropping the column.
+A configuration error - a bad name, an invalid type or length, a name collision, or a reference to a derived variable that is not defined earlier - stops the EDIT screen with a message naming the variable. A syntax error or an unknown column in the expression itself is caught when the view is compiled, and stops the screen with a message naming the table.
 
 !!! warning "Derived values can expose hidden data"
     An expression can reference a column that is hidden from the user by [Column Level Security](/column-level-security/), and the derived column will show the result. Configure derived variables with the same care as any other data exposure.