Errorcode when client wants to connect to "full" mountpoint should be changed

Hi,

the errorcode that clients receive when they want to connect to a mountpoint which has reached its maxlisteners limit should be changed from the current "404 File Not Found" to something more meaningful, since this is the same errorcode that gets returned when the requested mountpoint doesn't exist.

A possible solution to this would be returning 403 with a customized reason phrase like "Mountpoint full" (or similar) in the header (since this is the only thing (if any) that gets forwarded to the user with most clients). The HTTP 1.1 RFC (#2616) allows this:

6.1.1:

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The individual values of the numeric status codes defined for HTTP/1.1, and an example set of corresponding Reason-Phrase's, are presented below. The reason phrases listed here are only recommendations -- they MAY be replaced by local equivalents without affecting the protocol.

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