reftable: adapt write_object_record() to propagate block_writer_add() errors

reftable: adapt write_object_record() to propagate block_writer_add() errors

Previously, write_object_record() would flush the current block and retry
appending the record whenever block_writer_add() returned any nonzero
error. This forced an assumption that every failure meant the block was
full, even when errors such as memory allocation or I/O failures occurred.

Update the write_object_record() to inspect the error code returned by
block_writer_add() and flush and reinitialize the writer iff the
error is REFTABLE_ENTRY_TOO_BIG_ERROR. For any other error, immediately
propagate it.

If the flush and reinitialization still fail with
REFTABLE_ENTRY_TOO_BIG_ERROR, reset the record's offset length to zero
before a final attempt.

All call sites now handle various error codes returned by
block_writer_add().

Signed-off-by: Meet Soni <meetsoni3017@gmail.com>
Acked-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>