Provide less cryptic error for missing systemlib sections #9566
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If a new extension systemlib isn't wired up with the build system, its unit emitters and decls won't be embedded in the HHVM binary, causing a cryptic crash when trying to run Hack code ("Invalid varint value: too few bytes."). One then has to fire up a debugger to determine what systemlib is missing.
Instead, raise a more informative error if a specific systemlib was not found in the binary.
Split from #9564.