mk_core: mk_rconf: Be more verbose on indentation errors
mk_core: mk_rconf: Be more verbose on indentation errors

This extends the indentation verification logic to allow users to
distinguish the following three failure modes.

    [INPUT]
      Name dummy
     Tag 1

    => (1) Indentation level is too low

    [INPUT]
        Name dummy
    \tTag 1

    => (2) Incositent use of tab and space

    [INPUT]
      Name dummy
       Tag 1

    => (3) Extra indentation level found

Especially case (2) was somewhat difficult for users to recognize
what is wrong, because a) indentation looks fine on text editor,
b) but Fluent Bit is still complaining.

Signed-off-by: Fujimoto Seiji <ef879192bb1e41d519c82cf76440dc8e9a530806@clear-code.com>

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