Change to ObjectValidator

Validation errors are now wrapped in an ObjectValidationException rather
than being squashed into an IllegalArgumentException. This will allow
the REST controller to return a nicer result.
Experimental 'update metadata' in front-end

Just doing title for now, and no error handling as yet.
Added handling of common exceptions to REST ctrl
Some minor Java tidying

Added a few @ToResolve annotations, and made some Exception classes final
FE: Added rudimentary metadata save feedback

Not great, but it intercepts the things we care about so woo hoo
Added rudimentary validation to metadata form
Removed SCSS comp from backend aardvark-webapp

Current plans are to do all of this on the front-end only, so there's no
need for this at present.
Partial port of Processing Engine web layer

I've ported enough of this from Aardvark 0.15.x to get the basic
resource & validation serving working again.

Note that the fancy resource loader hasn't been ported fully yet, though.
Updated validation result JSP

* Now JSPX
* HTML5 taglib updated to accommodate this
Ported resource previewer from Aardvark 0.15

I've done a bit of tidying to consolidate the CSS & JS into a single
location. Some further work will be required to tidy this up.
Tidied up ported JS slightly
Some minor tidying + @ToResolve additions