multi: handle errors returned from socket/timer callbacks

The callbacks were partially documented to support this. Now the
behavior is documented and returning error from either of these

multi: handle errors returned from socket/timer callbacks

The callbacks were partially documented to support this. Now the
behavior is documented and returning error from either of these
callbacks will effectively kill all currently ongoing transfers.

Added test 530 to verify

Reported-by: Marcelo Juchem
Fixes #8083
Closes #8089

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