Sync: Refactor netrc parsing

Don't emit a message when the netrc file doesn't exist or couldn't
be opened.

Instead of trying to unpack the result of info.authenticators() and
catching the resulting TypeError when it's None, first store it to
a local and only unpack it if it has a value.

Also remove an unused import.

Change-Id: I5c404d91e48c261c1ab850c3e5f040c4f4c235cb