port rev. 2803: Detect invalid input to Code 39 encoder and reject it more explicitly
port rev. 2803:
Detect invalid input to Code 39 encoder and reject it more explicitly

git-tfs-id: [https://tfs.codeplex.com/tfs/TFS05/]$/ZXingNet/trunk;C86379

micjahn/ZXing.Net,bsoja/ZXing.Net,micjahn/ZXing.Net,bsoja/ZXing.Net,DHMechatronicAG/ZXing.Net,micjahn/ZXing.Net,DHMechatronicAG/ZXing.Net,DHMechatronicAG/ZXing.Net,bsoja/ZXing.Net