Add hints to all the "expect ___ field to appear on next page"

[jelaniwoods]

Several students this quarter got stuck b/c of error messages like this

and don't know how to proceed. A hint here would make it more explicit what is wrong

What actually would be better is to add more tests, checking for a label that has one matching input.

[jelaniwoods]

Similar to what was done in appdev-projects/omnicalc-1#25