Add basic xref apropos implementation to elisp-mode

* lisp/progmodes/elisp-mode.el (elisp--xref-find-definitions):
Filter out nil results.
(elisp--xref-find-apropos): New function.
(elisp-xref-find): Use it.

* lisp/progmodes/xref.el (xref--show-xrefs): Use `user-error'.