gh-532 concatenate errors in fixture importer (#533)
gh-532 concatenate errors in fixture importer (#533)

After changing the error response in gh-521, the importer would no
longer compile. This fixes this by simply concatenating the error
responses, if an error occurs.

To verify that it works the docker-compose setup has been run
succesfully:
  weaviate_weaviate_demo_importer_1 exited with code 0

Closes #532
weaviate/weaviate,weaviate/weaviate