reftable: propagate specific error codes in block_writer_add()

reftable: propagate specific error codes in block_writer_add()

Previously, functions block_writer_add() and related functions returned
-1 when the record did not fit, forcing the caller to assume that any
failure meant the entry was too big. Replace these generic -1 returns
with defined error codes.

This prepares the codebase for finer-grained error handling so that
callers can distinguish between a block-full condition and other errors.

Signed-off-by: Meet Soni <redacted>
Acked-by: Patrick Steinhardt <redacted>
Signed-off-by: Junio C Hamano <redacted>