Declare key-valid-p and readablep as error-free

* lisp/keymap.el (key-valid-p):
* lisp/subr.el (readablep): These functions are effect-free; declare
them correspondingly.  Note that readablep is not pure since its
return value depends on ambient state.

lisp/keymap.el

@@ -313,7 +313,7 @@ Modifiers have to be specified in this order:
 which is
 
    Alt-Control-Hyper-Meta-Shift-super"
-  (declare (pure t) (side-effect-free t))
+  (declare (pure t) (side-effect-free error-free))
   (let ((case-fold-search nil))
     (and
      (stringp keys)

lisp/subr.el

@@ -6820,7 +6820,7 @@ This means that OBJECT can be printed out and then read back
 again by the Lisp reader.  This function returns nil if OBJECT is
 unreadable, and the printed representation (from `prin1') of
 OBJECT if it is readable."
-  (declare (side-effect-free t))
+  (declare (side-effect-free error-free))
   (catch 'unreadable
     (let ((print-unreadable-function
            (lambda (_object _escape)