Don't bail out on :package-version in `defcustom'

Michael Sperber writes:

 > +:package-version
 > +        VALUE should be a list with the form (PACKAGE . VERSION)
 > +        specifying that the variable was first introduced, or its
 > +        default value was changed, in PACKAGE version VERSION.  This
 > +        keyword takes priority over :version.  The PACKAGE and VERSION
 > +        must appear in the alist `customize-package-emacs-version-alist'.

Will an error be signaled if not?  Where?

 > +        Since PACKAGE must be unique and the user might see it in an
 > +        error message, a good choice is the official name of the
 > +        package, such as MH-E or Gnus.
 >  :tag LABEL
 >          Use LABEL, a string, instead of the item's name, to label the item
 >          in customization menus and buffers.
 > @@ -458,6 +467,8 @@
 >  	 (custom-add-to-group value symbol type))
 >  	((eq keyword :version)
 >  	 (custom-add-version symbol value))
 > +	((eq keyword :package-version)
 > +	 (custom-add-package-version symbol value))
 >  	((eq keyword :link)
 >  	 (custom-add-link symbol value))
 >  	((eq keyword :load)
 > @@ -507,6 +518,10 @@
 >    "To the custom option SYMBOL add the version VERSION."
 >    (put symbol 'custom-version version))
 >  
 > +(defun custom-add-package-version (symbol version)
 > +  "To the custom option SYMBOL add the package version VERSION."

This really should document the format of VERSION.

 > +  (put symbol 'custom-package-version version))
 > +
 >  (defun custom-add-load (symbol load)
 >    "To the custom option SYMBOL add the dependency LOAD.
 >  LOAD should be either a library file name, or a feature name."
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