Simplify errors tests a bit so they don't all have to write a schema

[benjaminjkraft]

Some of the errors tests need to have their own schema, so the schema
can do something weird (or even be entirely invalid!). But most can
still share a schema. In this commit I have those indeed share a
schema, to avoid having to have a bunch of copies of mostly the same
schema. While doing so I noticed one error whose location wasn't very
useful, and fixed it.

Test plan: make check

I have:

• Written a clear PR title and description (above)
• Signed the Khan Academy CLA
• Added tests covering my changes, if applicable
• Included a link to the issue fixed, if applicable
• Included documentation, for new features
• Added an entry to the changelog

[csilvers]

Nice!