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/pageTimings.json

@@ -0,0 +1,18 @@
+{
+  "$id": "pageTimings.json#",
+  "$schema": "http://json-schema.org/draft-06/schema#",
+  "type": "object",
+  "properties": {
+    "onContentLoad": {
+      "type": "number",
+      "min": -1
+    },
+    "onLoad": {
+      "type": "number",
+      "min": -1
+    },
+    "comment": {
+      "type": "string"
+    }
+  }
+}