Fix GitHub workflow and revert Errorf behavior for 1 wrapped error

[phughes-scwx]

errors.Errof slight reversion

Complicates a bit (for the better) how the new errors.Errorf works:

• for 0 or 1 wrapped errors, we use errors.WithFrame around the returned error, so that printing with %+v will trigger viewing the frames;
• for >1 error we use errors.WithFrame on the wrapped errors.

Will make migrating from v0.1 to v0.2 not break this expectation, and is nice.

Also use errors.Errorsf in examples instead of fmt.Errorf and update some tests accordingly.

Clarify and add back in tests for errors.ErrorsFrom

Uncomment the tests, add docs, and update tests to work with the new, kind of nerfed, errors.ErrorsFrom.

Fix the workflow

Throw out the included boilerplate in favor of a working YAML file. Also:

• fix lint issues;
• fix tests assertions that failed on CI due to issues in how Ubuntu prints a stack frame name.