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/ecc-jsbn/README.md

@@ -0,0 +1,8 @@
+ecc-jsbn
+========
+
+ECC package based on [jsbn](https://github.com/andyperlitch/jsbn) from [Tom Wu](http://www-cs-students.stanford.edu/~tjw/).
+
+This is a subset of the same interface as the [node compiled module](https://github.com/quartzjer/ecc), but works in the browser too.
+
+Also uses point compression now from [https://github.com/kaielvin](https://github.com/kaielvin/jsbn-ec-point-compression).