Attribute values must be literals. The error you get when that doesn't
hold is pretty bad, e.g.:
```
unexpected expression: 1 + 1
```
You also get the same error if the attribute value is a literal, but an
invalid literal, e.g.:
```
unexpected expression: "foo"suffix
```
This commit does two things.
- Changes the error message to "attribute value must be a literal",
which gives a better idea of what the problem is and how to fix it. It
also no longer prints the invalid expression, because the carets below
highlight it anyway.
- Separates the "not a literal" case from the "invalid literal" case.
Which means invalid literals now get the specific error at the literal
level, rather than at the attribute level.
