The margin for error already seemed slim: The business-backed Massachusetts Taxpayers Foundation issued a report on Wednesday highlighting several uncertain aspects of Governor Charlie Baker’s spending plan for the next fiscal year. MTF said Baker’s plans to accelerate sales tax collections ($237 million in the budget), legalize sports betting ($35 million), and to impose high-cost drug penalties ($19 million) are just three of the items that remain up in the air, awaiting the Legislature’s approval.