walsender can ignore send failures in WalSndLoop

Hi,

There's a small issue in abfd192b, namely one of the error cases wasn't
changed when WalSndLoop was changed to be able to return.

I don't think this is likely to have any grave consequences, we'll
likely error out soon afterwards again.

Patch attached.

Greetings,

Andres Freund

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 Andres Freund                       http://www.2ndQuadrant.com/
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