Recover from input-loading errors in HTTP server

Catch and recover panics from within the input-source loading process
when running as an HTTP server.

The CLI version will still dump a big ugly panic without any attempt at
recovery because (a) it is not expected to run for more than one
visualization request and (b) it exists primarily as a testing
interface, so exposing more rather than less data about any problems
encountered directly to the user/script running the CLI interface would
be helpful.