Fix the modification to the symmetrised amplitudes that causes a segfault when there are pre-symmetrised contributions. Correct the error message for the covariant correction for spins > 4.

git-svn-id: svn+ssh://phab.hepforge.org/source/laurasvn/trunk@344 30227d83-a2f9-49e9-9e98-657c83b2c86a