config: make numeric parsing errors more clear

config: make numeric parsing errors more clear

If we try to parse an integer config argument and get a
number outside of the representable range, we die with the
cryptic message: "bad config value for '%s'".

We can improve two things:

  1. Show the value that produced the error (e.g., bad
     config value '3g' for 'foo.bar').

  2. Mention the reason the value was rejected (e.g.,
     "invalid unit" versus "out of range").

A few tests need to be updated with the new output, but that
should not be representative of real-world breakage, as
scripts should not be depending on the exact text of our
stderr output, which is subject to i18n anyway.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>