aur-rpc: Send aria2c messages to stderr
aur-rpc: Send aria2c messages to stderr

I noticed this line occasionally appearing in the output of `aur-sync`:

```
parse error: Invalid numeric literal at line 1, column 3
```

Digging deeper, here's what I found:

- In some occasions, during the dependency-resolution loop in
  `aur-deps-rpc`'s `chain()`, the list of packages sent to `aur-rpc`
  will be blank. This can currently be reproduced with `printf %s\\n
  python-pywavelets python-scikit-image python2-glog | aur deps-rpc -t`.

- `aur-rpc` will transform and pipe the URLs into `ariac2`, when it's
  installed.

- When its stdin (list of URLs) is empty, `aria2c` prints "No files to
  download." to **stdout**!

- `jq` then attempts to parse the above string, resulting in the parse
  error.

Clearly, a whole bunch of things are wrong here:

- `aria2c` should not print that message with `-q`

- `aria2c` really should be sending messages to stderr by default

- Perhaps we should avoid invoking `aur-rpc` completely when the
  package list is empty.

For now, just ask aria2c to send its messages to stderr, so at least
we don't attempt to parse its garbage.

AladW/aurutils,AladW/aurutils