Remove ambiguity from "not safe to write" compiler error message

[kidandcat]

Fixes #4290

With this change, the error becomes:

Error:
/Users/jairocaro-accinoviciana/issues-pony-4290/demo.pony:10:11: not safe to write right side to left side
    x.foo = foo
          ^
    Info:
    /Users/jairocaro-accinoviciana/issues-pony-4290/demo.pony:9:14: right side type: Foo ref
        let foo: Foo ref = Foo
                 ^
    /Users/jairocaro-accinoviciana/issues-pony-4290/demo.pony:10:5: left side type: X iso
        x.foo = foo
        ^

(First time contributing to ponyc + first time contributing in C) triple check the change please 😅

[SeanTAllen]

This should have a test.

[SeanTAllen]

@jemc @jasoncarr0 because i got turned around a little with this, can one of you review this as well?

[ponylang-main]

Hi @kidandcat,

The changelog - fixed label was added to this pull request; all PRs with a changelog label need to have release notes included as part of the PR. If you haven't added release notes already, please do.

Release notes are added by creating a uniquely named file in the .release-notes directory. We suggest you call the file 4299.md to match the number of this pull request.

The basic format of the release notes (using markdown) should be:

## Title

End user description of changes, why it's important,
problems it solves etc.

If a breaking change, make sure to include 1 or more
examples what code would look like prior to this change
and how to update it to work after this change.

Thanks.

[SeanTAllen]

This is likely to be lacking in meaning for a reader of the release notes who doesn't have your context.

Please include what the error message used to be, why that was vague, and how the new version of the message fixes the vagueness.