Fix option name in block incremental map errors.

The error for an invalid value in a block incremental map named the option with cfgParseOptionKeyIdxName(), which expects a key index, but the callers passed a group index. A repository configured as anything but repo1 was named as repo1. Use cfgOptionIdxName() instead, which takes the group index and maps it to the key.

The errors for the checksum size map also named the size map option since that option id was passed by mistake.

These options are internal so neither error was user-facing.