opam_installer.sh: fix wget emulation by cURL.

`curl -O` (aka `--remote-name`) always saves the file into the current directory and thus the `-o $TMP/$opamfile` part was ignored.

I prefer calling `curl` explicitly to using it to emulate `wget` (trying to be too smart is not a good engineering.)

`error()` is useless as long as `set -e` is in effect.