Create content: add better logging for unexpected errors

During testing of ESSENTIALS-1108 I came across an error due to a not yet implemented feature. The blog document has validators on not implemented fieldtypes. After the first step (document name etc) clicking on the "continue" button created the error.

The uses sees a nice message "unexpected error occured", but there is no javascript error nor any message in the server logs. So a developer/admin has no clue about what is the problem. The only indiction is a 500 response in a request that you can see in the network panel of the browser. The response's content is {"reason":"SERVER_ERROR"}. This is not really helpfull.

Please make sure there is a stacktrace logged that gives a clue for these kind of cases. (I know that the actual issue will be fixed, but any future unexpected error tracking will be made easier).