update-manager takes proxy settings from unreasonable places

[Jure Sah]

While trying to do a do-release-update today on Ubuntu 16.04 I hit an unusual snag ("No new releases found") which turned out to be caused by out of date proxy settings in Synaptic -- an apt package management GUI that was last relevant approximately 10 years ago in Ubuntu 9.04 .

Test case:
1. Have invalid proxy settings from 10 years ago in "/root/.synaptic/synaptic.conf"
2. Run release update either through GUI or using "do-release-upgrade" in bash
3. Receive error "No new releases found"

I have confirmed that once I deleted "/root/.synaptic/synaptic.conf", the distribution update shows up correctly.

I understand the consideration of this file was added to the release-updater code trying to resolve a bug, but it is really unreasonable to expect someone to be able to figure out this is to blame for a rejected distribution update in 2020.

At the very least include some debugging information.