lint: Fix 'check-texinfo-markup'.

Fixes a regression introduced in 5d8d8f3b.

* guix/scripts/lint.scm (check-description-style): When no exception is
  thrown in 'check-texinfo-markup', return the rendered description.

guix/scripts/lint.scm

@@ -146,11 +146,13 @@ (define (check-not-empty description)
   (define (check-texinfo-markup description)
     "Check that DESCRIPTION can be parsed as a Texinfo fragment.  If the
 markup is valid return a plain-text version of DESCRIPTION, otherwise #f."
-    (unless (false-if-exception (texi->plain-text description))
-      (emit-warning package
-                    (_ "Texinfo markup in description is invalid")
-                    'description)
-      #f))
+    (catch #t
+      (lambda () (texi->plain-text description))
+      (lambda (keys . args)
+        (emit-warning package
+                      (_ "Texinfo markup in description is invalid")
+                      'description)
+        #f)))
 
   (define (check-proper-start description)
     (unless (or (properly-starts-sentence? description)