500 error if no matching <chart_type>

[tym-xqo]

If a request URL specifies a <chart_type> value that doesn't match a provisioned chart, the app hits a KeyError exception trying chart = charts[chart_type] and barfs out a generic 500 message.

Consistency with Nerium would suggest that we should fall back to a default chart_type if no match is found. Another option might be to return some sort of "chart type not found" message.