Detect and handle errors fetching the latest version

This attempts to detect such errors, assuming that they appear with
suitable HTTP status codes, and raises useful errors for them.

Fixes iterative#59

src/utils.js

@@ -35,9 +35,14 @@ const download = async (url, path) => {
 const getLatestVersion = async () => {
   const endpoint = 'https://updater.dvc.org';
   const response = await fetch(endpoint, { method: 'GET' });
-  const { version } = await response.json();
-
-  return version;
+  if (response.ok) {
+    const { version } = await response.json();
+    return version;
+  } else {
+    const status = `Status: ${response.status} ${response.statusText}`;
+    const body = `Body:\n${await response.text()}`;
+    throw new Error(`${status}\n${body}`);
+  }
 };
 
 const prepGitRepo = async () => {