Pass codes not strings for Xapian::Error subclasses in remote protocol

[Olly Betts]

Currently the remote protocol passes the name of the error subclass in the remote protocol. It would be better overall to assign numeric codes and pass these because:

• We could have a switch on a compact range of integers in the client, rather than a series of string compares, which should be more compact and faster.
• Smaller message size, though this isn't a performance sensitive case.

The only downside I see is the small maintenance overhead of having to maintain an enumeration, but it isn't like we add or remove error classes very often at all.

Requires a major remote protocol bump, so milestone:1.3.0 (unless we require a major remote protocol bump for some other reason during 1.2.x).

[Olly Betts]

Fix in trunk r15664.