Improve error message for `arity mismatch`

[jfbastien]

I'm running test pr44942.c.s.wast from https://storage.googleapis.com/wasm-llvm/builds/git/wasm-torture-s2wasm-2718.tbz2 and the error message is just arity mismatch. It would be nice to also know which function (test1 in this case) and the declared parameter count versus how many arguments were given.

That's a bug in the toolchain, I'll fix it :-)

FWIW I just did something similar here:
WebAssembly/binaryen@4044cbd
But ocaml makes me sad :)

[rossberg]

That behaviour has been gone for some time.