Fix xpath library wrong query failures in case we continue supporting it

[ncordon]

In #423 we introduced a temporary fix to deal with incorrect queries in XPath (see #424 for context) and fail catching the panic and avoid crashing of clients.

In case we continue supporting XPath we should shift to fixing those issues in the xpath library itself (maybe forking it or doing PRs upstream) and fail with a Go error code instead of a panic.