vtfontcvt: Improve error message for unsupported DWIDTH

vtfontcvt requires that all glyphs are 1x or 2x a common width,
reporting for example "bitmap with unsupported DWIDTH 27 0 on line xxx"
if the font is expected to be 32 pixels wide.

Add the expected / permitted values to the error message to make the
issue more clear - for the same example,
"bitmap with unsupported DWIDTH 27 0 (not 32 or 64)".

Reviewed by: ziaee
Sponsored by: The FreeBSD Foundation