Histogram TypeError

- log an error to console if the wrong data type is passed to moz_chart for a histogram. Currently, a cryptic error from d3 pops up and the rest of the script fails to run.

- added http prefix to two source tags in index.html so I could test things on my machine. Not sure if there's a reason why they were missing the prefix.


That's to make the requests protocol-agnostic--our test server forces https, so we have d3.js hosted locally there. I wonder if we ought to pull d3.js from a cdn that supports https too, e.g. https://cdnjs.cloudflare.com/ajax/libs/d3/3.4.11/d3.min.js

@hamilton?