profiles: Use a &message error condition instead of 'error'.

* guix/profiles.scm (sexp->manifest): Use 'raise' instead of 'error'.

guix/profiles.scm

@@ -278,7 +278,8 @@ procedure is here for backward-compatibility and will eventually vanish."
                                   search-paths))))
            name version output path deps search-paths)))
     (_
-     (error "unsupported manifest format" sexp))))
+     (raise (condition
+             (&message (message "unsupported manifest format")))))))
 
 (define (read-manifest port)
   "Return the packages listed in MANIFEST."