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/cache.json

@@ -0,0 +1,21 @@
+{
+  "$id": "cache.json#",
+  "$schema": "http://json-schema.org/draft-06/schema#",
+  "properties": {
+    "beforeRequest": {
+      "oneOf": [
+        { "type": "null" },
+        { "$ref": "beforeRequest.json#" }
+      ]
+    },
+    "afterRequest": {
+      "oneOf": [
+        { "type": "null" },
+        { "$ref": "afterRequest.json#" }
+      ]
+    },
+    "comment": {
+      "type": "string"
+    }
+  }
+}