(a) Standard for Reversible Error. No judgment may be reversed on appeal on the ground that the trial court made an error of law unless the court of appeals concludes that the error complained of:
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(1) probably caused the rendition of an improper judgment; or
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(2) probably prevented the appellant from properly presenting the case to the court of appeals.
(b) Error Affecting Only Part of Case. If the error affects part of, but not all, the matter in controversy and that part is separable without unfairness to the parties, the judgment must be reversed and a new trial ordered only as to the part affected by the error. The court may not order a separate trial solely on unliquidated damages if liability is contested.