Check wasm optimized output file exists

Created by: ascjones

Possible improvement for the source of this error message here:

Suspect that the installed version of wasm-opt runs but doesn't produce the output file as expected.

I haven't reproduced it because my installation of wasm-opt is valid and produces a optimized file, however this is a useful check to make anyway, since we have no guarantees that do_optimization will write the optimized wasm fle.