Debugger Plugin: TensorBoard doesn't error out if gRPC --debugger_port is already used

[caisq]

To reproduce:

1. Start a tensorboard process with --debugger_port configured to a given port, say 6064.
2. In another terminal, start another tensorboard process with a different http --port, but the same --debugger_port value. Observe that the second process prints an C++ level error log line, but it doesn't really error out.

[caisq]

cc @chihuahua

[nfelt]

Hmm, does this happen on all platforms or just on Mac? Sounds maybe related to #1320 though I assume the socket logic here is within gRPC.

[caisq]

@nfelt I haven't tested on Macs yet. I observed this on a Linux laptop. Not sure what @chihuahua used when he first reported this issue.

[chihuahua]

Thanks for reporting, @caisq! This was on my ubuntu workstation.