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/assert-plus/CHANGES.md

@@ -0,0 +1,14 @@
+# assert-plus Changelog
+
+## 1.0.0
+
+- *BREAKING* assert.number (and derivatives) now accept Infinity as valid input
+- Add assert.finite check.  Previous assert.number callers should use this if
+  they expect Infinity inputs to throw.
+
+## 0.2.0
+
+- Fix `assert.object(null)` so it throws
+- Fix optional/arrayOf exports for non-type-of asserts
+- Add optiona/arrayOf exports for Stream/Date/Regex/uuid
+- Add basic unit test coverage