That does not excuse his yelling at you, but just so you understand his frustration. As for the Bible's qualifications for leaders, this should have been addressed by the pastor when you first offered to volunteer. But again, maybe he was so desperate to find someone he simply grabbed the first person who stepped forward without worrying about their qualifications. It would have been best if you expressed your doubts when you first had them rather than waiting till the last minute to express them. But that said, what the pastor should have done is had a serious conversation about what you felt your deficiencies were and how you could be supported so you could still do it.
Bottom line is, the situation does seem like it was handled poorly on both sides.
This was the first of many back slides for us. Some time went by and we were feeling convicted to get back to church but had no idea were to attend. My wife had attended a Christian school and said that there was a teacher she had that really made an impact on her life. So we gave him a call and explained our situation. He was a great source of inspiration and helped us on our feet once again.