try / catch hb's PlanckError-s, issue #39

In no case shall a python program die for an exception from external
extension.

There is no way to emit an exception in ufunc, thus we set the calculated
numbers to invalid values:
 for (long) [pixel index, usually], -1 is set
 for (double) [vector, angle], NAN is set.

Conflicts:
	healpy/test/test_pixelfunc.py
Adding comments to test_pixelfunc.py, issue #39
checking that invalid nside does not crash healpy, issue #39
added set float invalid status

currently fails with a link error,
needs some include or link flags
Revert "added set float invalid status"

This reverts commit f21ce9d595a121a2455038a70d46dd7427a6142e.
Using npy_set_floatstatus_invalid makes the code fail with a
linking error when imported, reverting this for now.
Merge pull request #77 from healpy/catchplanckerror

Catchplanckerror