Improve error output w/ bad args
Improve error output w/ bad args

dart-lang/markdown,dart-lang/markdown,tomyeh/markd,srawlins/dart-markdown,srawlins/dart-markdown,dpeek/dart-markdown,tomyeh/markd,dpeek/dart-markdown,dart-lang/markdown,srawlins/dart-markdown,tomyeh/markd