fusesoc.in: catch DependencyError from BackendFactory
fusesoc.in: catch DependencyError from BackendFactory

Signed-off-by: Franck Jullien <7ddcd52d897de62c0010070b24d1f61cf69e2d93@gmail.com>

lowRISC/fusesoc,lowRISC/fusesoc,olofk/fusesoc,olofk/fusesoc