fix: first try widget downloads with custom fetch

[kevineger]

Initially, I set out to fix a logging bug; if the HTTP attempt (no custom fetch) failed but the custom fetch succeeded, the HTTP error would still be logged. After making that fix, I came to the opinion that it likely makes more sense to first try the custom fetch and then fallback. Otherwise we're issuing a ton of redundant requests.

This change also:

• Removes some duplication by lifting the two private getWidgetScriptSource* methods to an abstracted downloadFile function.
• Simplifies the logging. Previously the errors included from ([httpclient|fetch]) and the relevant ex. This duplicated the context (httpclient vs. fetch). Since the inner error contains this info, we can use the same top level error message when we log.

With minimal context of the existing code, I can't easily determine... but for reviewers with more background:

1. When would using the custom fetch ever fail?
2. Do we even need the fallback, or would it make more sense to strictly rely on the custom fetch?
3. Is the fallback behaviour I'm proposing here desirable or would you still prefer we issue both requests simultaneously and I instead change this PR to only log errors if both fail?

I looked to write or modify a unit test, but the RemoteIPyWidgetScriptManager doesn't appear to have existing coverage. If reviewers feel the change necessitates a test, I can look to cover the component. Though IMO that's out of scope given the precedent.