Fix WARNS check return.

Catch the return value of the enclosed form and return it from `warns`
check.  Previously, simply returned T or NIL, which caused problems with
surrounding code.

src/check.lisp

@@ -266,10 +266,12 @@ is evaluated in a block named NIL, CONDITION-SPEC is not evaluated.
   Is like SIGNALS, but does NOT abort the execution of BODY upon the signal
 being raised."
   (let ((block-name (gensym))
-        (signaled-p (gensym)))
+        (signaled-p (gensym))
+        (normal-return (gensym)))
     (destructuring-bind (condition &optional reason-control reason-args)
         (ensure-list condition-spec)
-      `(let ((,signaled-p nil))
+      `(let ((,signaled-p nil)
+             ,normal-return)
          (block ,block-name
            (handler-bind ((,condition (lambda (c)
                                         (unless (typep c 'warning)
@@ -279,15 +281,16 @@ being raised."
                                                     :test-expr ',condition)
                                         (setf ,signaled-p t)
                                         (muffle-warning c))))
-             (block nil
-               ,@body))
-           (when ,signaled-p (return-from ,block-name t))
+             (setf ,normal-return
+                   (block nil
+               ,@body)))
+           (when ,signaled-p (return-from ,block-name ,normal-return))
            (process-failure
             ',condition
             ,@(if reason-control
                   `(,reason-control ,@reason-args)
                   `("Failed to signal a ~S" ',condition)))
-           (return-from ,block-name nil))))))
+           (return-from ,block-name ,normal-return))))))
 
 (defmacro finishes (&body body)
   "Generates a pass if BODY executes to normal completion. In