[WebProfilerBundle] Show error message in toolbar instead of JS confirmation

| Q                | A
| ---------------- | -----
| Bug report?      | no
| Feature request? | yes
| BC Break report? | no
| RFC?             | yes
| Symfony version  | 3.4

As suggested by @javiereguiluz in https://github.com/symfony/symfony/pull/23251#issuecomment-310048591, show a fake toolbar with an error message instead of the JS confirmation dialog when loading the toolbar fails.