emacs: Gracefully handle wrong package names passed to 'guix-edit'.

Fixes <http://bugs.gnu.org/22933

>.

* emacs/guix-main.scm (package-location-string): Rewrite to handle the
case where 'packages-by-name' returns the empty list.

Co-authored-by: Mathieu Lirzin <mthl@gnu.org>

emacs/guix-main.scm

@@ -954,10 +954,11 @@ (define (delete-generations* profile generations)
 
 (define (package-location-string id-or-name)
   "Return a location string of a package with ID-OR-NAME."
-  (and-let* ((package  (or (package-by-id id-or-name)
-                           (first (packages-by-name id-or-name))))
-             (location (package-location package)))
-    (location->string location)))
+  (and=> (or (package-by-id id-or-name)
+             (match (packages-by-name id-or-name)
+               (()              #f)
+               ((package _ ...) package)))
+         (compose location->string package-location)))
 
 (define (package-source-derivation->store-path derivation)
   "Return a store path of the package source DERIVATION."