Indicate to user the reason for failure of a URL pull job

[sentry]

If a URL that the user enters 404s (or has some other non 200 status code) the user gets a relatively obscure error message.

Sentry Issue: HUB-15K

RuntimeError: Error when fetching .<REDACTED URL>: 404

File "jobs/base/job.py", line 191, in run
result = self.do(*args, **kwargs)
File "jobs/pull/init.py", line 69, in do
files = pull_func(source=source, path=path, secrets=secrets)
File "jobs/pull/http.py", line 26, in pull_http
raise RuntimeError(f"Error when fetching {url}: {response.status_code}")