Handle scripts that don't return a 200 status code

This was already being handled for async scripts, but for sync scripts, we'd
log the error then proceed to try and execute the body (which would be some
error message).

This allows the header_callback to return a boolean to indicate whether or not
the http client should continue to process the request or abort it.

src/browser/webapi/net/Fetch.zig

@@ -89,7 +89,7 @@ pub fn deinit(self: *Fetch) void {
     }
 }
 
-fn httpHeaderDoneCallback(transfer: *Http.Transfer) !void {
+fn httpHeaderDoneCallback(transfer: *Http.Transfer) !bool {
     const self: *Fetch = @ptrCast(@alignCast(transfer.ctx));
 
     if (transfer.getContentLength()) |cl| {
@@ -133,6 +133,8 @@ fn httpHeaderDoneCallback(transfer: *Http.Transfer) !void {
     while (it.next()) |hdr| {
         try res._headers.append(hdr.name, hdr.value, self._page);
     }
+
+    return true;
 }
 
 fn httpDataCallback(transfer: *Http.Transfer, data: []const u8) !void {