fusesoc.in: Change error string formatting

Signed-off-by: Franck Jullien <franck.jullien@gmail.com>
fusesoc.in: catch exception on configure and add_cores_root

Signed-off-by: Franck Jullien <franck.jullien@gmail.com>
provider: raise exception on unknown/missing provider

Signed-off-by: Franck Jullien <franck.jullien@gmail.com>
provider: url/opencores raise exception on _checkout errors

Signed-off-by: Franck Jullien <franck.jullien@gmail.com>
Improve error output printing style

Signed-off-by: Franck Jullien <franck.jullien@gmail.com>