[unifi] Various stability improvements (#14249)

[unifi] Various stability improvements (#14249)

* [unifi] Various stability improvements

- Fixed bug, causing nullpointer, in devices not reporting voltage/ampere values on PoE ports.
- Added some null checks to avoid having null pointer exceptions.
- Handled timeout exception that is in ExecutionException.
- Improved handling case where server returns data, but it's not from the api, and therefor can't be parsed.
- Improved adding additional text to the error messages by adding the reported exception message too.

Signed-off-by: Hilbrand Bouwkamp <hilbrand@h72.nl>