iproto: make iproto_write_error() blocking in debug

iproto_write_error() used to be blocking until the commit
4dac37a6 (iproto: remove
iproto_write_error_blocking())
Actually, it should block until the error is written to socket, because
some tests (vinyl/errinj.test.lua, for example) rely on that.

Do not make iproto_write_error() blocking in release builds for safety
reasons, as stated in commit above. But make it blocking in debug for
testing sake.

Part-of #6034