Improve playpen error handling

[whipsch]

There was no case for handling an error returned by the playpen web application itself. This commit ensures that future mishaps like #107 are more visible to users and are more likely to get reported due to the hard error message, instead of nothing happening.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @brson (or someone else) soon.

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. The way Github handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

Please see CONTRIBUTING.md for more information.

[brson]

Thanks!