Log errors if request fails in http cache

[tsauerwein]

Closes #491

[sbrunner]

@tsauerwein When we to an HTTP request who it failed, is it cached?
For me it shouldn't...

[tsauerwein]

Responses for requests with a status code other than 200 are also cached. The users of the cache (e.g. layers) can decide what they do in such a case. But that is not the point of this PR.

[sbrunner]

But that is not the point of this PR.

Not directly but it's good to understand the context :-)

The users of the cache (e.g. layers) can decide what they do in such a case.

I try to find some more informations here: http://mapfish.github.io/mapfish-print-doc/layers.html without any success... How is it working?

[tsauerwein]

The users of the cache (e.g. layers) can decide what they do in such a case.

I try to find some more informations here: http://mapfish.github.io/mapfish-print-doc/layers.html without any success... How is it working?

By "users of the cache" I mean the layer classes which get the request responses from the cache and decide for example if a pink image is rendered or the print fails. Users of MFP can configure this behavior with failOnError, see http://mapfish.github.io/mapfish-print-doc/layers.html

[tsauerwein]

Merging because no functional change, this PR is only adding logging statements.
@sbrunner, if you want to change the behavior of the error handling, I'd propose to do this in a new PR.