In WCS.all_world2pix, doctests, don't re-raise exceptions

The doctests catch an exception and then print some of the exception's properties. However, doctest evidently can't handle re-raising an exception in try-except block, or can't handle print statements inside the except block.

This patch removes the `raise` inside the `except` block and re-enables the doctest, which now passes (at least on Mac OS X Mavericks).