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/har-schema/lib/header.json

@@ -0,0 +1,20 @@
+{
+  "$id": "header.json#",
+  "$schema": "http://json-schema.org/draft-06/schema#",
+  "type": "object",
+  "required": [
+    "name",
+    "value"
+  ],
+  "properties": {
+    "name": {
+      "type": "string"
+    },
+    "value": {
+      "type": "string"
+    },
+    "comment": {
+      "type": "string"
+    }
+  }
+}