Improve error messages for OCD-IDs about civics_cdf_validator

[miano]

Addressed 1 and 2 in the commit above. 3 should be caught by the schema rule since the restriction in the schema is case sensitive(see below). Relaxed it to the rule to be case-insensitive nonetheless.

<xs:simpleType name="IdentifierType">
        <xs:restriction base="xs:string">
            <xs:enumeration value="fips" />
            <xs:enumeration value="local-level" />
            <xs:enumeration value="national-level" />
            <xs:enumeration value="ocd-id" />
            <xs:enumeration value="state-level" />
            <xs:enumeration value="other" />
        </xs:restriction>
</xs:simpleType>