Pipefail for PanPipe

[Chris Warburton]

In repos.md, there's a script like this:

```{.unwrap pipe="bash | pandoc -f html -t json"}
set -e
...
```

Bash hit a failure in the script, due to a missing file, and aborted
with an error message, as we'd expect from using `set -e`. All well and
good.

Unfortunately, PanPipe ignored the error and carried on, generating an
incomplete page and allowing the whole site generation process to
continue.

This is probably because we're using a pipe in `pipe=`, and bash has the
stupid default behaviour of only treating a pipeline as failing if the
*last* component failed; since `pandoc` didn't fail, only `bash` did, no
error was reported.

We should update PanPipe to use the `pipefail` option.