T02 support and better error handling

[klonfish]

This PR adds support for the T02 printer (via bluetooth only). It also adds a printer option to set the number of feed lines at the end of the print for tearing off the label with different defaults for both models. This was necessary as the previous number of feed lines (2) was too few for the T02.

Additionally, it contains some fixes done by @darkmattercoder for better error handling. They e.g. allow to use the driver only via bluetooth and gracefully ignore USB if the required lib is not installed.