reftable: add error related functionality

reftable: add error related functionality

The reftable/ directory is structured as a library, so it cannot
crash on misuse. Instead, it returns an error code.

In addition to signaling errors, the error code can be used to signal
conditions from lower levels of the library to be handled by higher
levels of the library. For example, in a transaction we might
legitimately write an empty reftable file, but in that case, we want to
shortcut the transaction.

Signed-off-by: Han-Wen Nienhuys <hanwen@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>