Validation succeeds after addError

## Version

3.15.0

## Test Case

//somewhere in code
this.changeset.addError('someField', 'You failed a complex validation');

//in the submit method
await this.changeset.validate();
if (this.changeset.isValid) {
//
}

## Expected Behavior

I would expect the changset to fail validation.

## Actual Behavior
the error is cleared from changeset.errors and the changeset passes validation.