Don't show 500 errors if connection to data API fails

Some of the metrics on the instructor dashboard are computed by the new
analytics data pipeline. Don't show 500 errors if the connection to the
pipeline API fails. Instead display that the data is not available and
log an error.

AN-1476
Merge pull request #4301 from rocha/display-na-on-instructor-dashboard

Don't show 500 errors if connection to data API fails