Improve error message and ctests for input encoding cp858.

Improve error message and ctests for input encoding cp858.

Encoding cp858 supported by only some iconv variants.
Most users will want to change their "encoding" setting instead
of installing/recompiling "iconv" to support this legacy encoding.

ctests are likely will fail with either "vanilla" or "enhanced"
iconv and test a situation that is unlikely to change generally,
so we ignore this test now by default.