Improve error if unable to open meta.json file

Closes jameseh96#3.

Previously, if the file could not be opened an exception would be
thrown from in nlohmann json, with a misleading message.

Check the state of the ifstream before handing off to parse json.

Author: jameseh96

src/pdu/block/index.cc

@@ -175,6 +175,10 @@ void Index::load(std::shared_ptr<Resource> res) {
 
     {
         std::ifstream metaF(metaPath.string(), std::ios::in);
+        if (!metaF.good()) {
+            throw std::runtime_error("Failed to open \"" + metaPath.string() +
+                                     "\" when trying to parse index meta");
+        }
         meta = nlohmann::json::parse(metaF);
         metaF.close();
     }