Improve resilience if diff fetch fails or .osc.gz file is corrupted

[b1tw153]

If the download of a .osc.gz file is interrupted, this can leave an incomplete file in the diff directory. Then, when apply_osc_to_db.sh attempts to use this file, it will hang and fail to complete the update. At this point, the script will output the following error:

gzip: stdin: unexpected end of file
Reading XML file ...Parse error at line NNNN:
unclosed token

To recover from this scenario, the broken .osc.gz must be re-downloaded manually. The fetch_osc.sh script will not automatically replace it. Then, all of the components must be restarted including the dispatcher processes. If apply_osc_to_db.sh is restarted without restarting the dispatcher, it will still hang and fail to complete the update.

Two fixes would be good. The fetch_osc.sh script could have better protection against interruptions. And the apply_osc_to_db.sh script could handle corrupted .osc.gz files better.

[b1tw153]

I think this issue should be reproducible by manually truncating a .osc.gz file before it is processed.