osmosis-update.sh needs to be able to handle mid-execution disruption

[donalhunt]

While testing this new script, I discovered that /data/imports/bin/trim_osc.py was not executable which meant the whole import was failing. However the state.txt file was updated so re-running the script would have resulted in missing data.

The script needs to do a better job of cleaning up after itself (especially during failures).

[donalhunt]

Will look at this as part of my current work on bringing tileservers back online. We may retire the script in favour of imposm3 though.