bdev: add and use two utility functions

This patch adds two utility functions for raising BlockDeviceError
exceptions and for running functions while ignoring this error. Most of
the manual “raise errors.BlockDeviceError” cases are converted to
_ThrowError, as this makes the code clearer.

We also change most of the DRBD error messages to include the minor
number because with the parallel execution of commands it's not longer
possible to identify the failed DRBD just from the timestamp, and the
minor number can be mapped back to the instance easier.

Reviewed-by: ultrotter