[bfss storage] Improve fspath() error message.
This changeset improve the error message of fspath when the argument is not
managed by a BytesFileSystem storage.
#! /bin/sh -e#if [ "$1" = remove -o "$1" = upgrade ]; then if which install-docs >/dev/null 2>&1; then install-docs -r cubicweb-doc fifi#DEBHELPER#exit 0