* lisp/net/eudc.el (eudc--plist-member): Signal a more
informative wrong-type-argument instead of a generic error
(bug#58531#19, bug#58720).
* test/lisp/net/eudc-tests.el (eudc--plist-member)
(eudc-plist-member, eudc-plist-get, eudc-lax-plist-get):
Update tests accordingly.