Fix #43 (xbps-install: unhelpful message for invalid dependency)

If a package that is going to be installed or updated contains invalid
dependencies return ENXIO and XBPS_STATE_INVALID_DEP xbps state to clients.

This improves the error messages returned to the clients when such
condition happens.

bin/xbps-install/transaction.c

@@ -224,6 +224,8 @@ install_new_pkg(struct xbps_handle *xhp, const char *pkg, bool reinstall)
 		} else if (rv == ENOTSUP) {
 			fprintf(stderr, "No repositories  "
 			    "currently registered!\n");
+		} else if (rv == ENXIO) {
+			fprintf(stderr, "Package `%s' contains invalid dependencies, exiting.\n", pkg);
 		} else {
 			fprintf(stderr, "Unexpected error: %s\n",
 			    strerror(rv));