5b2: name the violation in every error message, and the escape a claimed line takes

src/markdown/text-directive.ts

@@ -14,6 +14,6 @@ export function readTextDirective(span: DirectiveSpan): Read<string> | undefined
   if (span.name !== name) return undefined
   const spelled = readSoleStringAttribute(span, name)
   if (spelled.fault !== undefined) return spelled
-  if (!whitespaceRun.test(spelled.value)) return { fault: unsupportedNodeShape(`${name} spells one run of spaces and tabs, or one run of newlines`) }
+  if (!whitespaceRun.test(spelled.value)) return { fault: unsupportedNodeShape(`${name} spells one run of spaces and tabs, or one run of newlines: this one spells neither`) }
   return spelled
 }