pyke/pyke.h, key.c: Rename `INDEXERR' to `MAPERR'.

The name `INDEXERR' was pointlessly confusing, given the existing
`IXERR' which actually raises Python's `IndexError'.  It's important,
though, that `KEYERR' be reserved for Catacomb's `KeyError' exception,
which really is an error about (cryptographic) keys.  So `MAPERR' it is.