Some error messages use the indefinite article:
"expected a bytes-like object, %.200s found"
"cannot use a bytes pattern on a string-like object"
"cannot use a string pattern on a bytes-like object"
but others don't:
"expected string instance, %.200s found"
"expected str instance, %.200s found"
Messages tend to be abbreviated, so I think that it would be better to just omit the article.
I don't think that the error message "bad repeat interval" is an improvement (Why is it "bad"? What is an "interval"?). I think that saying that the min is greater than the max is clearer.