private-json-hooks.cc: deal with EPIPE

While running our CI we noticed that sometimes we see an error
from the new json hooks code. The error message is:
```
E: Could not read response to hello message from hook [ ! -f /usr/bin/snap ] || /usr/bin/snap advise-snap --from-apt 2>/dev/null || true: Broken pipe
```
when purging the snapd package which provides the hook. This indicates
that we should probably also consider EPIPE not an error (just like
we do for ECONNRESET). This PR does exactly this.

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