Error 152 should not be fatal

[briangmilnes]

Invalid --warn_error setting: cannot change the error level of fatal error 152

In order to support compilation in a directory hierarchy and particularly in emacs,
one wants to put in possibly existing include directories. Otherwise each buffer's
settings would have to be specific to the location of the file.

So in Forge, I have src/fstar/fst (and it works without this useful tree by default in just
src). It has an src/fstar/proofs for expensive extrinsic proofs (thanks to Jonathan's
makefiles for showing the need of this). And test has the same src tree but needs to
include from the parent src.

It should warn and be supressable to allow compilation not to be overrun with errors on each file compilation.