Show InputDecoration.errorText when the field is disabled in M2 - #12589

_InputDecoratorDefaultsM2.errorStyle resolved the error text color to
Colors.transparent for the disabled state. The subtext row is still laid
out for the error, so a disabled field with an errorText reserved space
below the input but painted nothing, leaving an unexplained gap.

The Material 3 defaults already paint the error text with the error
color in every state. This change removes the disabled special case from
the Material 2 defaults so both align, and the space that is reserved
for the error is used by the visible error text.

Ported from flutter/flutter#191662, which is blocked by the Material code freeze.

Fixes flutter/flutter#67409