errors should indicate if they came from gocql directly

All errors which are defined in gocql should indicate that they came from gocql by having a `gocql:` prefix in the error. This will help users identify where errors come from and potentially be clearer as to why.

Search the repository for all errors defined in gocql and inspect how their messages are constructed. Confirm that each such error consistently begins with the `gocql:` prefix, then run the relevant existing tests or the repository test suite to verify the behavior.

Written by the indexing model from the issue text.