`ExistingRefError.Error()` omits the ref name and type

doltdb.ExistingRefError is returned when a caller-provided name collides with an existing ref. Its Error() explicitly returns a "already exists", a bare string.

https://github.com/dolthub/dolt/blob/56fc3f4f292598b644df4397d5ef1799a5883c07/go/libraries/doltcore/doltdb/errors.go#L158-L162

The type carries a Ref field that identifies the conflict. Branch creation already recovers this detail out of band, but tag and workspaces do not.

Error() should include the ref, for example: ref 'refs/heads/x' already exists. Updating 'Error()` will change the message tag and workspace callers and test assert against, so they'll have to be updated with their own wrappers.