driver pigcs: check the minimum speed when changing the speed

The SMOController doesn't support changing speed, but various speeds were allowed.
Also raise a useful exception.

Update the microscope files which were forcing the speed.
WARNING: this might cause error when starting odemisd if the microscope file has not been fixed.
model: Convert exception messages to ascii

no unicode, it causes troubles sometimes.
Also improve message for axis speed.
cli: display original error message when an exception happens

improves understanding of what went wrong for the user