bug#23975: 25.0.94: defcustom error message is wrong when :type field has a :match attribute

[Robert Weiner]

Given a defcustom like:

(defcustom bounded-num 999
  "Positive, bounded number"
  :type '(integer :match (lambda (widget value) (and (integerp value)
(> value 0)
      (< value 1000)))))

When this variable is customized and a value of -5 is entered, the
match function fails
and the error signaled is:

  (error "This field should contain an integer")

which is wrong and not helpful.  Instead the error should display what
the match function is and that the value failed to match.

Secondarily, it would be nice if the type were checked before the match
function were applied so that one did not need to add the (integerp
value) test into the match function.

Bob

In GNU Emacs 25.0.94.1 (x86_64-apple-darwin13.4.0, NS appkit-1265.21
Version 10.9.5 (Build 13F1603))
 of 2016-05-17 built on builder10-9.local
Windowing system distributor 'Apple', version 10.3.1404
Configured using:
 'configure --with-ns '--enable-locallisppath=/Library/Application
 Support/Emacs/${version}/site-lisp:/Library/Application
 Support/Emacs/site-lisp''

Configured features:
NOTIFY ACL LIBXML2 ZLIB TOOLKIT_SCROLL_BARS NS

Important settings:
  value of $LANG: en_US.UTF-8
  locale-coding-system: utf-8-unix

Major mode: Custom