Fallback for chart failure

[quadrophobiac]

This will trigger any time a JSON object with count value 0 has been returned by API.
This is a very small patch but should cover edge cases where data for an endpoint has no values within the last 30 days. I reasoned this was the best solution to this because anything else would involve changing the logic of several methods in metrics_helpers.rb

datetime_path()
get_single_metric()
get_metric_range()

the latter two of these are likely to be refactored with the work entailed in #173. At present the date tab retrieves the earliest possible date for a metrics endpoint, so I think directing users to that tab and including some text where the plotly chart would usually render is a sufficient workaround.

[coveralls]

Coverage remained the same at 88.196% when pulling 075de03 on patch-graceful-failure-for-plotly-graphs into 4641e34 on master.