Better UX when needed file not found

[pkgw]

Cf. issues like #30 or #33. When a file is just missing from the bundle, it would be good to give the user more guidance as to what to do. Namely.

1. That you can work around the problem by just downloading it and sticking it in the current directory, if you know where to find it.
2. That if you think the bundle should have contained the file, that you should file an issue with the bundle.

You could even imagine adding a metadata file to the bundle that includes an issue URL, so that in a hypothetical world with different bundles that people can choose, you can customize the URL that they see.

[rekka]

This looks like a tricky problem, since not all files the engine requests are necessary, some are just optional configuration files. After an error happens, tectonic might need to parse the engine stdout for things like file ... not found, etc., to detect that the error happened because of a missing file.