Changes

typechecker: fix an internal error due to wrong exception

`Ctype.mcomp` was raising an errortrace when two types were incompatible
in two rare cases, whereas the function was specified to raise the
`Incompatible` exception. This was fine for internal uses of `mcomp`
within `Ctype`, because call to `mcomp` went through `mcomp_for` that
transformed the `Incompatible` exception into an errortrace. However,
this leads to internal errors for others use of `Ctype.mcomp` that were
only expecting an `Incompatible` exception and not an errortrace.

This simple fix replaces the two raises of errortrace by a raise of
`Incompatible`.