improve Celery file system error handling

[Ed McDonagh]

Celery currently crashes effectively if a file cannot be written or deleted.

[Ed McDonagh]

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

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[Ed McDonagh]

Added try-except around CT xlsx and NHSBSP mammo csv temporary file creation. Refs #112

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[Ed McDonagh]

Adding errors filter to pass to template. Refs #112

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[Ed McDonagh]

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.

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[Ed McDonagh]

Added errors table to export template. Refs #112.

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[Ed McDonagh]

Replaced message of file io error message with update to progress, which is then displayed on exports template. Refs #112.

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[Ed McDonagh]

Added try-except-message to progress field to the csv export routines. Need to copy to mammo NHSBSP csv. Need to try and get a more useful error message. Refs #112.

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[Ed McDonagh]

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.

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[Ed McDonagh]

Applied better error status message exception to fluoro and CT csv and to CT xlsx. Refs #112.

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[Ed McDonagh]

Applied same try-except OSError-except code as for the csv exports and tested. Also tested with no file-permission errors, and everything still works. Refs #112.

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[Ed McDonagh]

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

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[Ed McDonagh]

Celerly file handling errors are now caught and displayed to the user for action. Fixes #112.

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[Ed McDonagh]

Added ref #112 to the changes log.

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