Internal errors in Syscall.CallAsync are always silent

[dbarnett]

Since the Syscall.CallAsync implementation runs vim --remote-expr … in a subshell and ignores the exit code and output, any errors in forwarding the results back into vim are silently ignored. For example, any problems with the shell command we build like quoting/escaping bugs would result in the handler just never getting called even if the original command itself succeeded. That would be insanely hard for callers to debug.

We should figure out a way to capture those error details and bubble them up.

[dbarnett]

Also swallows errors thrown in the callback, I think.

[dbarnett]

Fixed it to shout any errors that occur in the callback itself. I don't think we're going to be able to do much about errors in the shell and the like, but I think we'll call this "fixed" anyway. We'll have a more robust blessed option once we land native job support (#184 for vim, #125 for neovim).