Improved error message for cases of invalid or missing namespaces.

Since grizzly eats the entity from the jersey 2 responses, I couldn't easily write an integration test.

You can test this using curl:
curl -v -H "Content-Type: text/turtle" -X PUT --data-binary "@u.txt" "http://localhost:8080/rest/test/te/st/12/3/test123"

where u.txt is:
<http://localhost:8080/rest/test/te/st/12/3/test123> <http://fedora.info/definitions/v4/rels-ext#isPartOf> <http://localhost:8080/rest/test/sa/mp/le/:b/sample_batch_id> .

You can test a similar case by changing u.txt to:
<http://localhost:8080/rest/test/te/st/12/3/test123> <http://fedora.info/definitions/v4/rels-ext#isPartOf> <http://localhost:8080/rest/test/sa/mp/le/unregistered:b/sample_batch_id> .