mergeArms doesn't handle broken fibres in mergeLsfs

Now that pfsArms have broken fibres (PIPE2D-720), the assumption in mergeLsfs that assumes that all fiberIds are in warpLsf is invalid.
It is not obvious that the obvious fix of checking for the presence of the fibre is correct, or whether the problem should be pushed upstream. It's also not possible to check that the problem is indeed a broken fibre as pfsConfig isn't available