Remove stderr output channel

[dbarnett]

Even though vroom supports a stderr output channel, it's pretty much useless since vim itself doesn't distinguish between stdout and stderr when using system(), and tools like maktaba#syscall# that use redirection to catch stderr can't pay attention to the actual stderr stream received anyway.

We should just remove the stderr channel and point people to the command output channel instead unless we can find a valid use for stderr or change it to work natively with maktaba#syscall#'s stderr handling or something.