Fix Event Log Error Processing

[joannekoch]

ar16 has a missing elog file - an empty csv file is generated and no error is thrown.

ar61a has an erroneous character in its elog file and cannot be parsed. This error is displayed: Not Found: /api/nut/ar61a which is not the correct error.

in parse_elog change read_csv to:
try:
df = pd.read_csv(elog_path, dtype={
CAST: str
})
except Exception as e:
raise ValueError('event log file cannot be parsed {}'.format(elog_path))

in workflow_response change to:
filename = workflow.filename()
try:
df = workflow.get_product()
return dataframe_response(df, filename, extension)
except DataNotFound as e:
raise Http404(str(e))
except ValueError as e:
raise Http404(str(e))
except Exception as e:
raise Http404(str(e))