More Changset Validation

[blackboxlogic]

The API got some new limits here and here. Check the Capabilities client side before sending a changeset.

This is low priority, will consider it if we're making other changes already.

[HarelM]

Since this error will be returned from the OSM server (I hope) I'm not sure there's a real need to introduce logic here...?
Just a thought, you might have something in mind though.