error message from virtual modules doesnt point to the right executable

[avsm]

With a port of tcpip to use dune-master virtual libraries, it all works except that if an executable doesnt define an implementation, the error message points to somewhere else in the compilation stack.

e..g with this

(executables
 (names ping)
 (libraries cmdliner logs logs.fmt tcpip.icmpv4-socket))

the error message from dune is

$ dune build @all
Error: No implementation found for virtual library "tcpip-checksum" (_build/default/src/tcpip_checksum).
      -> required by library "tcpip.icmpv4" in _build/default/src/icmp

It would be nice if the error message pointed to the "root cause" which is the executable clause, not the dependent library in tcpip.icmpv4.

(PS: virtual modules rock! hurrah! The tcpip PR is at mirage/mirage-tcpip#384)