Handle missing ACE data in make_timelines.py

[taldcroft]

Dev/Arc task warning from Oct 24 2012. At the time ACE was not supplying data for about 10 hours.

'''
WARNING - '/proj/sot/ska/dev/share/arc/make_timeline.py --data-dir=/proj/sot/ska/dev/data/arc' returned non-zero status: 256
Traceback (most recent call last):
File "/proj/sot/ska/dev/share/arc/make_timeline.py", line 615, in
main()
File "/proj/sot/ska/dev/share/arc/make_timeline.py", line 273, in main
avg_flux = get_avg_flux(ACE_RATES_FILE)
File "/proj/sot/ska/dev/share/arc/make_timeline.py", line 113, in get_avg_flux
'AVERAGE (expect one)'.format(ACE_RATES_FILE, len(lines)))
ValueError: /data/mta4/www/ace.html file contains 0 lines that start with AVERAGE (expect one)
'''

[taldcroft]

As noted by @Gregg140 this issue ends up blocking all timeline processing so that the tabular data and timeline plot effectively stops updating unless ACE data are available.