Migration: Add Exception mapping from Java SDK 2.x to 3.x

Currently, we provide a lot of detail on how to smoothly migrate the Java SDK 2->3.

However, some parts are missing, for example, some exceptions which have changed, sometimes drastically.

I would suggest to add an exception migration section.

Exceptions of Java core 2.0 (for SDK 3). https://docs.couchbase.com/sdk-api/couchbase-core-io-2.0.0/com/couchbase/client/core/error/package-summary.html

for example, 

1. DocumentDoesNotExistException was changed to DocumentNotFoundException.
2. CASMismatchException was changed to CasMismatchException