Only throw for specific special built-in errors.

* gash/shell.scm (sh:exec-let): Do not throw on special built-in
errors.
* gash/built-ins/break.scm (main),
gash/built-ins/continue.scm (main),
gash/built-ins/return.scm (main),
gash/built-ins/shift.scm (main): Throw on error.

gash/built-ins/break.scm

@@ -35,7 +35,7 @@
      (match (string->positive-integer arg)
        (#f (format (current-error-port)
                    "gash: break: argument must be a positive integer~%")
-           EXIT_FAILURE)
+           (throw 'shell-error))
        (n (set-status! 0)
           (sh:break (1- n))
           (format (current-error-port)
@@ -43,4 +43,4 @@
           EXIT_SUCCESS)))
     (_ (format (current-error-port)
                "gash: break: too many arguments~%")
-       EXIT_FAILURE)))
+       (throw 'shell-error))))

gash/built-ins/continue.scm

@@ -35,7 +35,7 @@
      (match (string->positive-integer arg)
        (#f (format (current-error-port)
                    "gash: continue: argument must be a positive integer~%")
-           EXIT_FAILURE)
+           (throw 'shell-error))
        (n (set-status! 0)
           (sh:continue (1- n))
           (format (current-error-port)
@@ -43,4 +43,4 @@
           EXIT_SUCCESS)))
     (_ (format (current-error-port)
                "gash: continue: too many arguments~%")
-       EXIT_FAILURE)))
+       (throw 'shell-error))))

gash/built-ins/return.scm

@@ -35,7 +35,7 @@
      (match (string->exit-status arg)
        (#f (format (current-error-port)
                    "gash: return: argument must be a number from 0 to 255~%")
-           EXIT_FAILURE)
+           (throw 'shell-error))
        (n (sh:return n)
           (format (current-error-port)
                   (string-append "gash: return: no function "
@@ -43,4 +43,4 @@
           EXIT_SUCCESS)))
     (_ (format (current-error-port)
                "gash: return: too many arguments~%")
-       EXIT_FAILURE)))
+       (throw 'shell-error))))

gash/built-ins/shift.scm

@@ -44,8 +44,8 @@
          (format (current-error-port)
                  "~a: shift: Invalid option ~s.~%"
                  (car (program-arguments)) n-string)
-         EXIT_FAILURE))))
+         (throw 'shell-error)))))
     (_ (format (current-error-port)
                "~a: shift: Invalid options ~s.~%"
                (car (program-arguments)) args)
-       EXIT_FAILURE)))
+       (throw 'shell-error))))

gash/shell.scm

@@ -131,10 +131,7 @@ environment variable bindings @var{bindings}."
                                ((name . value)
                                 (setvar! name value)))
                              bindings)
-                   (let ((exit-val (apply proc args)))
-                     (unless (= exit-val EXIT_SUCCESS)
-                       (throw 'shell-error))
-                     (set-status! exit-val))))
+                   (set-status! (apply proc args))))
           (and=> (getfun name)
                  (lambda (proc)
                    (save-variables-excursion (map car bindings)