Avoid an NPE when parsing LogEvent responses

[squaremo]

We assume that if the upstream we're posting events to responds with
Content-Type: application/json, it must be a helpful error
(*github.com/weaveworks/flux/errors.Error). However, it's possible
to get any old JSON (even no JSON), and in that case, we'd trip
oursevles up by creating a helpful error with a nil pointer, leading
to a panic later.

[rade]

Have we seen this in the wild? #949 and #958 perhaps?

[squaremo]

Have we seen this in the wild? #949 and #958 perhaps?

Not sure. I noticed it after adding a List resource to my test repo, rather than a CRD. It was provoked by the upstream server sending a 400 Bad Request in response to the posted sync event, and maybe that happens for CRDs too.