Revert "fix(mango): GET invalid path under _index should not cause 500

[rnewson]

This reverts commit c1195e4.

This wasn't quite right. handle_index_req has several separate resources handled within it which each support a different set of http methods.

We should send 405 for a resource that exists but doesn't support the request's method.
We should send 404 for a resource that doesn't exist at all.

Reverting this gives us a 500 / function_clause, which is at least honest (the server has made an error) but we should circle back and send better errors.

Noting that;

1. extra paths for _all_docs gives the desired 404 but for a silly reason (it matches the clause that looks for attachments, and you can't make a doc called _all_docs to attach anything to)
2. extra paths for _changes are ignored (and the _changes request works as expected) because handle_changes_req doesn't insist on a pattern for path_parts

In conclusion, we handle invalid paths in request handlers inconsistently, which ironically frees us to do the right thing in handle_index_req