paftools.js prints errors to stdout rather than stderr

This makes it unfriendly to scripting and pipelines.

```
% paftools.js sam2paf >/dev/null
[Exit 1]
```
vs

```
% paftools.js sam2paf 2>/dev/null
Usage: paftools.js sam2paf [options] <in.sam>
Options:
  -p      convert primary or supplementary alignments only
  -P      convert primary alignments only
  -L      output the cs tag in the long form
[Exit 1]
```