Can not catch server startup errors

[adam-thomas-privitar]

Describe the bug
Currently, createServer calls server.listen but this method always resolves, even if there is a problem such as the port being taken. This means ultimately these exceptions land as unhandled exceptions as there is no way to handle them.

To Reproduce

1. Start prism-cli on a certain port.
2. In another terminal start one on the same port.

Expected behaviour
I'd expect the promise to reject, which would allow those of us using createServer directly to handle the scenario. It would also allow prism-cli to respond gracefully without causing an unhandled exception.

Screenshots
If applicable, add screenshots to help explain your problem.

Environment (remove any that are not applicable):

• Library version: 3.3.3
• OS: OSX Catalina