config: improve error message for boolean config

config: improve error message for boolean config

Currently invalid boolean config values return messages about 'bad
numeric', which is slightly misleading when the error was due to a
boolean value. We can improve the developer experience by returning a
boolean error message when we know the value is neither a bool text or
int.

before with an invalid boolean value of `non-boolean`, its unclear what
numeric is referring to:
  fatal: bad numeric config value 'non-boolean' for 'commit.gpgsign': invalid unit

now the error message mentions `non-boolean` is a bad boolean value:
  fatal: bad boolean config value 'non-boolean' for 'commit.gpgsign'

Signed-off-by: Andrew Klotz <agc.klotz@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>