Prevent executing a query when no connection exists

no ref

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I am not sure whether this is the best course of action, but it prevents errors like `cannot read property _cid of undefined.` from appearing and provides a clean error message. 

This is related to issues at Ghost (https://ghost.org/forum/installation/10207-error-cannot-read-property-__cid-of-undefined-at-runner_sqlite3-anonymous/) where either the file name for SQLite3 is missing or there are connection troubles with other databases.

A different solution would be to provide a `knex.checkConnection()` method that returns whether knex can interact with the database before sending off queries. Suggestions very welcome!