Consider renaming `unresolved-attribute` as `missing-attribute`

[jack-mcivor]

I think the latter is easier to understand and aligns with possibly-missing-attribute.

I'm not sure if there is a reason that "unresolved" is more correct or preferrable for a different reason. There are also other *unresolved* errors - I think currently:

• possibly-unresolved-reference
• unresolved-attribute
• unresolved-global
• unresolved-import
• unresolved-reference

[carljm]

Thanks for filing this! I do find "unresolved" to be awkward wording here, and I think "missing" would be clearly better in at least three of these cases: "missing-attribute", "missing-global", "missing-import".

"missing-reference" doesn't make sense, since it's not the reference that is missing but the referent. I think undefined-name and possibly-undefined-name would be better?