An invalid netrc file does not produce a warning

[mathialo]

I recently migrated one of our projects from poetry to uv, and one thing that made me stumble was a syntax error in my netrc file. Uv would just silently ignore the error and move on as if the netrc file did not exist, which made it a bit hard to figure out what was going on.

I think a better approach would be if the Netrc::new() call returns a parsing error, to log a warning about it before ignoring it, to let users know why the netrc file is being ignored.

[zanieb]

Seems reasonable, but we'd want to be careful to only display this once and only if we needed credentials.