Rephrase error related to sample data initialization
Rephrase error related to sample data initialization

Now that the initialize step is fully implemented, the source of E923 is
typically missing or improperly converted/formatted data rather than a
bug in spaCy, so rephrase the error and message and remove the prompt to
open an issue.

spacy-io/spaCy,honnibal/spaCy,explosion/spaCy,honnibal/spaCy,spacy-io/spaCy,explosion/spaCy,honnibal/spaCy,explosion/spaCy,spacy-io/spaCy,spacy-io/spaCy,explosion/spaCy,explosion/spaCy,spacy-io/spaCy,spacy-io/spaCy,honnibal/spaCy,explosion/spaCy