Fleshed out 404 page. Added funky cat picture.

[nilsnh]

Here's the start of a pull request on issue #7 . Please give feedback.

[snorremd]

Error-page looks better, nice cat picture.

One of the main problems we want to address with #7 is that the same kind of error is displayed for all types of errors. For instance we want to inform the user when they submit an event outside the geographic boundary defined by the config.

I don't know if this should be addressed in a separate merge request though.