API: correctly propagate $@ with nested locks

API: correctly propagate $@ with nested locks

PVE::Cluster::cfs_lock_file sets $@ and returns undef for all errors,
including when $code dies. PVE::Tools::lock_file runs $code inside an
eval as well, so just setting $@ is not enough when nesting these two
types of locks.

re-die with the inner error to actually propagate error messages and
fail instead of proceeding. this triggered (probably among other cases)
when attempting to join an existing cluster without specifying all
needed links.

Signed-off-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>