bug#19212: 24.3; Signaled error doesn't show `error-message'

[dfsr@riseup.net (Diogo F. S. Ramos)]

According to (info "(elisp) Error Symbols"), the following sequence of
forms should signal an error with the string "A new error: x, y":

 (put 'new-error
      'error-conditions
      '(error my-own-errors new-error))
 (put 'new-error 'error-message "A new error")
 (signal 'new-error '(x y))
 
Instead, I got the following in the debugger:

 Debugger entered--Lisp error: (new-error x y)
   signal(new-error (x y))
   eval((signal (quote new-error) (quote (x y))) nil)
   eval-last-sexp-1(nil)
   eval-last-sexp(nil)
   call-interactively(eval-last-sexp nil nil)

I evaluated each form inside `*scratch*' using C-x C-e.