Add helper functions to print to log/print to stderr

While reviewing https://github.com/coder/modules/pull/329 I noticed we didn't print any errors to stderr. I suggest we add helper functions for this:

```sh
# lib
error() {
echo "ERROR: $@" >&2
exit 1 # (optional)
}

log() {
echo "$@"
}
```

Then use them where appropriate:

```sh
log "Installing package..."
install || error "Failed to install package"
```

// cc @matifali