CA-73263: Improve error raised after failure to parse plugin output.
CA-73263: Improve error raised after failure to parse plugin output.

This error is raised whenever the plugin output is not parseable as an
XMLRPC response - this is the case whenever the plugin prints anything
to stdout besides the final response so the error can be quite
misleading.

Signed-off-by: John Else <37b74501daa612e48fea94f9846ed4ddeb6a8f07@citrix.com>

simonjbeaumont/xcp-rrdd,koushikcgit/xcp-networkd,djs55/squeezed,sharady/xcp-networkd,koushikcgit/xcp-rrdd,johnelse/xcp-rrdd,johnelse/xcp-rrdd,djs55/xcp-rrdd,simonjbeaumont/xcp-rrdd,sharady/xcp-networkd,djs55/xcp-networkd,koushikcgit/xcp-networkd,koushikcgit/xcp-rrdd,robhoes/squeezed,koushikcgit/xcp-rrdd,djs55/xcp-rrdd,djs55/xcp-networkd