Add more meta information & error handling

[nbrinckm]

We discussed yesterday that it could be helpful to have some more error handling support.

I think my pull request doesn't solve this exact problem, but I think it is still helpul to add the exception report in the jobs table (in case the WPS executation failed).
And I also add created_at fields, so that we know when the jobs/outputs where created.

[arnevogt]

Certainly makes sense to store the exception if a job ultimately fails.

I think this will only log the final exception when a job fails; don't know if there is an easy way to store every exception for every retry. In issue #98 it seems to be the case that the WPS is stuck after n retries.

[MichaelLangbein]

Looks great to me! Can be merged if you're content with it.
Will need need to read those in the frontend as well. I've added a note to issue #81 .

[nbrinckm]

Certainly makes sense to store the exception if a job ultimately fails.

I think this will only log the final exception when a job fails; don't know if there is an easy way to store every exception for every retry. In issue #98 it seems to be the case that the WPS is stuck after n retries.

It will set the exception stacktrace in the moment that we consider the wps call as failed anyway.

In case we have the retry for the wps call itself, it can be the last one after the retries (so network related in this case).
If it would be one of the exceptions that we don't consider for retrying we would store the stacktrace of those exception.

It it is stuck in the retry somehow & we don't consider the job as failed so far, then we still don't save this exception stacktrace. We simply don't get them for the moment - so storing it is not possible yet.