Convert C++ Public API Error handling to use couchbase::error

Following up from the work done in CXXCBC-463, to be in-line with the RFC, we should convert the current error return type from specific error context types (e.g. key_value_error_context) to a generic couchbase::error.

This work will be split up into multiple tickets due to its scope. 

Additional work will also have to be done in the FIT performer after the main work in the SDK is done, this will be a non-backwards compatible with previous SDK versions.