Don't exit process when requesting a failed build

If the most recent build failed and an HTTP request came in, the history middleware would throw an uncaught exception causing the process to exit. Instead, catch and ignore.

lib/tasks/server/middleware/history-support/index.js

@@ -49,7 +49,16 @@ class HistorySupportAddon {
 
     app.use(async (req, _, next) => {
       try {
-        const results = await watcher;
+        let results;
+        try {
+          results = await watcher;
+        } catch (e) {
+          // This means there was a build error, so we won't actually be serving
+          // index.html, and we have nothing to do. We have to catch it here,
+          // though, or it will go uncaught and cause the process to exit.
+          return;
+        }
+
         if (this.shouldHandleRequest(req, options)) {
           let assetPath = req.path.slice(baseURL.length);
           let isFile = false;