Ignore broken entries instead of 500ing

If an entry doesn't exist anymore on the filesystem (but it's returned by a query on psql, coming from a situation like https://github.com/gbdev/homebrewhub/issues/93), it should be ignored and removed from the returned results, instead of making the API call return a server error.

Start by tracing the API call that loads entries from the psql-backed query and then checks whether each entry still exists on the filesystem. Reproduce a query result containing a missing entry; done means that entry is omitted from the returned results and the API no longer returns a 500 error.

Written by the indexing model from the issue text.