raise ReferenceError when given a bad reference
add integration tests for batch import rollbacks
batch import controller handles reference errors
add reference validation to streaming import
Add a helper to check whether a record exists
Rename 'extract_refs' into something more obvious, and move the 'external ref' checking to its own helper
Remove strays puts
Patch up test data to avoid using external repository links for URIs
Merge pull request #469 from hudmol/check-references-before-importing

Check references before importing