Add failure description (from errno) when failing to open device

[ola1olsson]

Maybe I should throw an FileOpenError instead? Anyway, not having this error message cost me 30 minutes today and I don't want anyone else feel as stupid as I did when I noticed it was a permission problem.

[shumatech]

I need to rework the error reporting out of the open() methods in the SerialPort classes so as not to expose higher level code to its implementation. I'm thinking either an exception or a last error cause method.