mwdocgen.php: Exit with exit code of passthru(doxygen)

Previously it just naturally exited which means even if the
doxygen bin doesn't exist, it'll return "success".

Bug: 46151
Change-Id: Ie1e3d956ce5d3e1f28c7c829bb57596113f07c4a
Merge "mwdocgen.php: Exit with exit code of passthru(doxygen)"