Added checks to make sure the specified track exists

Added precondition checks to validate input files and paths to return more informative error messages as per #2

node_modules/ajv/scripts/prepare-tests

@@ -0,0 +1,12 @@
+#!/usr/bin/env sh
+
+set -e
+
+mkdir -p .browser
+
+echo
+echo Preparing browser tests:
+
+find spec -type f -name '*.spec.js' | \
+xargs -I {} sh -c \
+'export f="{}"; echo $f; browserify $f -t require-globify -t brfs -x ajv -u buffer -o $(echo $f | sed -e "s/spec/.browser/");'