Improve error message when server is unreachable (#288)

This change improves the error message for when the remote file you're trying to access is unreachable. This can happen if you misspelled the URL, the server is down for some reason or if you don't have access to the internet.

Author: selbekk

bin/loaders/index.js

@@ -7,7 +7,14 @@ const loadFromHttp = require("./loadFromHttp");
 async function load(pathToSpec) {
   let rawSpec;
   if (/^https?:\/\//.test(pathToSpec)) {
-    rawSpec = await loadFromHttp(pathToSpec);
+    try {
+      rawSpec = await loadFromHttp(pathToSpec);
+    } catch (e) {
+      if (e.code === 'ENOTFOUND') {
+        throw new Error(`The URL ${pathToSpec} could not be reached. Ensure the URL is correct, that you're connected to the internet and that the URL is reachable via a browser.`)
+      }
+      throw e;
+    }
   } else {
     rawSpec = await loadFromFs(pathToSpec);
   }