levelup constructor without callback causes unhandled rejection

Using v4.0.1 of levelup and leveldown. The following command causes an unhandled rejection:

levelup(leveldown('/path/to/nonexistent/file'))

UnhandledPromiseRejectionWarning: OpenError: IO error: /path/to/nonexistent/file: No such file or directory
....
    at (node:69500) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). (rejection id: 1)

This happens because the final line in the LevelUP constructor, this.open(callback), is a floating promise.

[vweevers]

Aye. The unhandled rejection is somewhat consistent with the previous behavior of emitting an error event, but the difference is you can’t catch it now. Perhaps we should change this line:

https://github.com/Level/levelup/blob/c56f863e41f1b00216837963be34d4ba74458b19/lib/levelup.js#L57

To:

this.open(callback || function (err {
  if (err) self.emit('error', err)
})

[vweevers]

Fixed in 4.3.1