add test for file exists, fix messaging
add test for file exists, fix messaging

astashov/dartdoc,jakemac53/dartdoc,a14n/dartdoc,guillermooo-forks/dartdoc,astashov/dartdoc,srawlins/dartdoc,guillermooo-forks/dartdoc,guillermooo-forks/dartdoc,srawlins/dartdoc,d2m/dartdoc-2015,jakemac53/dartdoc,keertip/dartdoc,srawlins/dartdoc,jakemac53/dartdoc,keertip/dartdoc,jakemac53/dartdoc,d2m/dartdoc-2015,d2m/dartdoc-2015,astashov/dartdoc,srawlins/dartdoc,astashov/dartdoc,d2m/dartdoc-2015,dart-lang/dartdoc,a14n/dartdoc,keertip/dartdoc,guillermooo-forks/dartdoc,dart-lang/dartdoc,a14n/dartdoc,dart-lang/dartdoc,keertip/dartdoc,dart-lang/dartdoc