Better typescript definitions + expose DatabaseError

[jacobdr]

• Export more precise typings for data types
  This enabled the following:

  

• Expose the database error so that users can use instanceof checks
  This enabled the following:

  

[erayhanoglu]

Hi Jacob,

1. Exposing DatabaseError is ok.
2. Setting DataTypeOIDs as const, i am not sure. We can not be sure that DataTypeOIDs is a complete list of all oids. So i think that it should allow adding items via augmentation. This is why we didn't use enum. Did you test that your code do not prevent augmentation?