Test for a free port instead of guessing from a restricted range

[dwalluck]

Fix retry not handling RuntimeException.

Record the last exception.

[rnc]

Looks like the code was changed and commented out by @jdcasey with a comment about a race condition

[dwalluck]

Looks like the code was changed and commented out by @jdcasey with a comment about a race condition

Let me list the issues I found with the current code and why I changed anything at all since I didn't put a thorough commit message:

1. It was restricting the port range seemingly unnecessarily. I was actually hitting an exception (inside Docker), which is how I noticed. The idea is to pass port 0, and let the OS pick the port.
2. When it hit the exception, it wasn't handling it. It throws BindException (IOException) when it can't bind to the port. The code catches IOException, so it looks like it handles it, except the actual exception thrown appears to be inside a RuntimeException from Undertow.
3. The actual exception/cause was being lost, which is why I store it and then rethrow.