Color inspector responses for 200/400/500 messages

[doolittle]

We show a red border on some inspector errors, but we should also do so for 400/500 error to make it blindingly obvious to the user an error occurred.

A just-completed response should have a green border, then fade down to grey.

When a cell is running, the entire border should turn blue, and the cell should appear to be disabled (i.e. a light blue overlay).