#42 Do request-status error code handling

[Shane Landrum]

We need to be able to generate a number of
error and success codes for the REQUEST-STATUS field---
see the request_status field in ETJ.pm. They have
numerics, with possible short explanations, and I think
we should have a master list of the numerics and what
they mean, possibly correlated with short strings. See
my comments in ETJ.pm for more detail.

I suspect that however LWP handles error code
generation and interpretation, we could do similarly.

[Shane Landrum]

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This is also an issue in Freebusy.pm.