Fix panics when viewing source of failed build (redux)

[jyn514]

This reverts all the changes from #519 except for actually fixing the panic. I tested this with directory listings as well as individual files:

• If no files exist, gives a 200 with an empty page (not ideal but better than the panic + blank screen from before). This can be tested with e.g. pam-sys 1.0.0-alpha1.
• If a directory listing exists, show that. Can be tested with most crates, e.g. rand 0.7.2.
• If a file exists, show that (no logic changed)

r? @pietroalbini

[pietroalbini]

Can we still show a not found page when None is returned?

[jyn514]

I think this only shows a 404 for binaries and not libraries, but I'd appreciate you testing locally too after last time 😆

[pietroalbini]

This looks great!