Merge branch 'ms/reftable-block-writer-errors'

Give more meaningful error return values from block writer layer of
the reftable ref-API backend.

* ms/reftable-block-writer-errors:
  reftable: adapt write_object_record() to propagate block_writer_add() errors
  reftable: adapt writer_add_record() to propagate block_writer_add() errors
  reftable: propagate specific error codes in block_writer_add()