Give Fetch/XHR breadcrumbs a log level based on status code

Higher order tracking issue: https://github.com/getsentry/team-sdks/issues/100

### Description

It came up that the breadcrumbs we create for 4xx and 5xx fetch requests are pretty tame and don't stick out. We can sorta fix this by changing the log level of the breadcrumbs based on the response status code:

- 5xx should probably be error
- 4xx should probably be warning/error