Rollup merge of #66263 - guanqun:make-error-explicit, r=alexcrichton
Rollup merge of #66263 - guanqun:make-error-explicit, r=alexcrichton

make the error message more readable

When I type it wrong e.g. `--stage --bless`, missing a number here, this change would make it more explicit what's going wrong here.

aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust