Feature request: Allow Retriever to work without the CREATE permission if the database already exists

[ethanwhite]

Via email I've received a request to support a situation where the database already exists, but the user does not have the permission to CREATE new databases. This is definitely an edge use case, but one that clearly can come up with shared resources.

A quick look at engine.py suggests that we can probably accomplish this fairly easily by adding a try-catch in create_db. The trick will be failing gracefully in the case where the user doesn't have permission to create and the database doesn't already exist.

[bendmorris]

Trying to continue anyway after a CREATE fails is a good default; it certainly shouldn't hurt anything. I've made the change.