WIP: option to fall through to middleware chain on source 404 (#248)

[aral]

By default, the proxy handles all requested routes on the given context(s), even if they are 404. This makes sense for routes that are actually handled by the target but there is a use case where it would be useful to be able to fall-through to the middleware chain if the target cannot handle the request. This would allow, for example, for a server’s functionality to be extended by another server.

This merge request adds a fallthrough option. When present, any 404 errors fall through to the middleware stack of the server by calling next().

Context: #248

[ghost]

DeepCode's analysis on #30855c found:

• ❌ 0 critical issues. ⚠️ 0 warnings and 3 minor issues. ✔️ 0 issues were fixed.

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