UART backend: check access permissions

[koenenwmn]

Currently, when the user has no write permissions the communication fails with:

glip: gb_uart_open: Cannot open device /dev/ttyUSB0
glip: glip_open: Cannot open backend (rv=-1)
ERROR: Cannot open backend

It would be nice to check the permissions first and give a more descriptive message about the cause of the problem.

[imphil]

Better check the permissions after the opening failed to give a more helpful error message, but avoiding corner cases where our checking fails but the device can still be opened.