libcdecl: Tweak invalid character error from scanner.

libcdecl: Tweak invalid character error from scanner.

By adjusting how we format the error message, the format string is
changed to be completely identical to a format string used by the
parser error reporting, which avoids some wasted code space in
the library.

Furthermore, make some tweaks to the invalid character pretty-printing
which seems to let GCC generate a bit more compact code.