packages: 'package-field-location' really catches 'system-error.

This had been wrong since forever (i.e., 2013).

* guix/packages.scm (package-field-location): Catch 'system-error, not
'system.

guix/packages.scm

@@ -351,7 +351,7 @@ (define (goto port line column)
 
   (match (package-location package)
     (($ <location> file line column)
-     (catch 'system
+     (catch 'system-error
        (lambda ()
          ;; In general we want to keep relative file names for modules.
          (with-fluids ((%file-port-name-canonicalization 'relative))