better error message when contract imports unimplemented host functions

[cdetrio]

When trying to import an unimplemented host function using wabt, the only error message is:

geth_evmc_test.go:143: evmc: contract validation failure

[axic]

Checked wabt and I'm not sure it is easy to do this :(

However we could extend chisel to display unknown/invalid symbols.

[aarlt]

hmm.. i only looked briefly in the evmc interface, so probably im wrong.. but is there any concept of returning "error messages" (in contrast to simple return codes) back from an evmc implementation? i was also not able to find geth_evmc_test.go, so i dont know the full context of this issue..

[axic]

but is there any concept of returning "error messages" (in contrast to simple return codes) back from an evmc implementation?

Yes, but problem we had here is getting specific messages from wabt in the first place.

Hopefully #540 improved it.