A court may not grant injunctive relief in a labor dispute: if the plaintiff has failed to comply with each obligation imposed by law that is involved in the labor dispute; or except as provided in subsection (b) of this section, if the plaintiff has failed to make every reasonable effort to settle the labor dispute: by negotiation; or with the help of available dispute resolution mechanisms, governmental mediation, or voluntary arbitration. If irreparable injury is threatened, a court may grant injunctive relief before another tribunal acts to settle the labor dispute.