Celery currently crashes effectively if a file cannot be written or deleted.
Added try except routine around temporary file creation in main csv export routines. Need to replicate for CT xlsx and for NHSBSP Mammo csv. Can test routines still work, but not sure if can test the new function as I don't know where the temp files are stored. Refs #112
Wrapped xlsx save in a try to catch file errors. Added messages import to show error message to user, but will probably end up using django-async-messages as this is a celery job. Or not use messaging at all. Refs #112.
Added in exception for OSError to get more useful feedback about what the problem was. Also stopped trying to redirect to a URL, as this is an asynchronous task and that doesn't make sense - it was also sending the celery server into a sea of red error messages. Refs #112.
Log file creation errors are now caught for size imports, the error category is passed to the view and the view displays them. Abort, delete error import and delete don't seem to work, probably unrelated. Refs #112