Fix website error when saturations.csv does not exist

Checks that 'saturations.csv' exists before calling it in web/main.py. Closes #88 (closed).

Before: https://ldas-jobs.ligo-wa.caltech.edu/~lockloss/index.cgi?event=1239585012

After: https://ldas-jobs.ligo-wa.caltech.edu/~yannick.lecoeuche/index.cgi?event=1239585012

(Not sure if I like the way the 'no saturations' messages are displayed, but that's for another merge request)