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.
Whichever is fine with me - as long as it doesn't break stuff locally while
we develop / the deployment of the examples page.

https is always preferred, though.