write out json attribute binding correctly

and remove unsused imports
check client provided workspace and store exist

it would be possible to create the importcontext with non-existing
workspace and target store otherwise causing failure later
move style computation ahead of other checks

when using a persistent store, it's possible that an upload
that is missing a SRS will not have the featuretype in it's metadata
map since that is marked transient to prevent serialization issues.

by computing the style first, we ensure that this is done while the
featuretype is still present

also add an exception if we cannot handle the resourceinfo class