py/objstr: In format error message, use common string with %s for type.

This error message did not consume all of its variable args, a bug
introduced long ago in baf6f14d.  By fixing
it to use %s (instead of keeping the string as-is and deleting the last
arg) the same error message string is now reused three times in this format
function and gives a code size reduction of around 130 bytes.  It also now
gives a better error message when a non-string is passed in as an argument
to format, eg '{:d}'.format([]).