raft: introduce RaftError

Last piece of src/box used in raft code was error.h. It was added
to be able to raise ClientErrors. To get rid of it the libraries
usually introduce their own error type available from
src/lib/core. Such as CollationError, SwimError, CryptoError.

This patch adds RaftError and removes the last box dependency from
raft code.

Part of #5303