bug#54976: 29.0.50; Eshell breaks when unknown entries in eshell-modules-list

[Sean Whitton]

Hello,

Suppose you put this in your init.el:

    (add-to-list 'eshell-modules-list 'eshell-elecslash)

because you want to use the new em-elecslash.el module I'm working on.

Then if you try to M-x eshell on an older version of Emacs which lacks
em-elecslash, Eshell will fail to initialise at all.

It is easy add a guard (when (locate-library ...)) or similar, but this
is not possible if you have added eshell-elecslash using the customise
interface.  In that case, by enabling the module, youq just make your
init unusable with older Emacs.

It would be better to print a warning, say, rather than error.

-- 
Sean Whitton