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/lib/refs/data.json

@@ -0,0 +1,17 @@
+{
+    "$schema": "http://json-schema.org/draft-07/schema#",
+    "$id": "https://raw.githubusercontent.com/epoberezkin/ajv/master/lib/refs/data.json#",
+    "description": "Meta-schema for $data reference (JSON Schema extension proposal)",
+    "type": "object",
+    "required": [ "$data" ],
+    "properties": {
+        "$data": {
+            "type": "string",
+            "anyOf": [
+                { "format": "relative-json-pointer" }, 
+                { "format": "json-pointer" }
+            ]
+        }
+    },
+    "additionalProperties": false
+}