netmd track rename: Correctly send the length of the old title

The "WRITE INFO BLOCK" AV/C command used to replace the track title
requires the host to provide the number of bytes to replace. Sharp
cares about that value and rejects it if more bytes are to be replaced
than are currently present, so send the correct value here.
(python) netmd: Fix expected exception type

On (old) devices that don't support the instruction to disable DRM,
that call raises an exception. Catch the type that really gets thrown
("unimplemented command") instead of "command rejected (but generally
valid)".