Fix bug in and rename throw_nil, warn_nil.

In warn_nil, the return value was undefined on exception -- I think.

While informative, the names throw_nil and warn_nil don't work very
well in conditionals:

  if (warn_nil<f>(...)) g();

sounds like g should be called if f returns nil and a warning is
issued. But it is actually the other way around; g is called if f is
successful.

  if (prefer<f>(...)) g();

sounds less like that.

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