Added assets service method AssetsExist(), which returns whether the given list of assets exist.

This method is used to optimize sending assets with embedded assets: e.g., when a Hypergrid visitor takes an item into the inventory.
Fixed the "Update Asset" handler: it was looking for the Asset ID in the wrong parameter.

This doesn't actually matter because the "Update Asset" operation isn't implemented in AssetsServer. But still, the handler should do the right thing...
Fixed last-resort sending of error response (HTTP 500) when an error occurs while handling a request.

The previous code didn't actually send the response, so the caller was stuck until the timeout (100 seconds).
Better string matching when searching for REST handlers: must match an entire path component (ending with '/' or a similar character).

For example, these should match: "/assets" and "/assets/12345", but these shouldn't match: "/assets" and "/assets_exist".