Bug 952206 - Abort execution when a required make file fails to remake; r=bsmedberg

Previously, errors during make file making (either the main make file or
an included make file) were ignored. This behavior diverged from GNU
Make, which aborted execution if the make file was required and ignored
the failure if it was optional.

This patch changes pymake to abort if the make file is required, brining
its behavior inline with GNU make.

--HG--
extra : rebase_source : f9a3f4c54273930616f4a8e41dafc463f363c891
extra : amend_source : f02236c3127602357f8f82a66d021144b3cdf3a0