[Bug 1848974] [NEW] update-notifier fails on ALL sources if ONE source is bad

[seahen]

Public bug reported:

Update-notifier doesn't work at all if *one* package source gives a list
that can't be parsed. It should still be able to work on packages from
*other* sources. After seeing the error in the system tray icon, I ran:

```
$ sudo apt-get update

E: Encountered a section with no Package: header
E: Problem with MergeList /var/lib/apt/lists/adoptopenjdk.jfrog.io_adoptopenjdk_deb_dists_disco_main_binary-amd64_Packages
E: The package lists or status file could not be parsed or opened.
```
apt-get successfully processed all but one of the package lists, so I would have expected update-notifier to do so as well.

I had to manually file this bug, because this error also prevents
ubuntu-bug update-notifier from working (see attached).

** Affects: update-notifier (Ubuntu)
     Importance: Undecided
         Status: New

** Attachment added: "Screenshot_2019-10-20_18-49-34.png"
   https://bugs.launchpad.net/bugs/1848974/+attachment/5298740/+files/Screenshot_2019-10-20_18-49-34.png

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https://bugs.launchpad.net/bugs/1848974

Title:
  update-notifier fails on ALL sources if ONE source is bad

Status in update-notifier package in Ubuntu:
  New

Bug description:
  Update-notifier doesn't work at all if *one* package source gives a
  list that can't be parsed. It should still be able to work on packages
  from *other* sources. After seeing the error in the system tray icon,
  I ran:

  ```
  $ sudo apt-get update

  E: Encountered a section with no Package: header
  E: Problem with MergeList /var/lib/apt/lists/adoptopenjdk.jfrog.io_adoptopenjdk_deb_dists_disco_main_binary-amd64_Packages
  E: The package lists or status file could not be parsed or opened.
  ```
  apt-get successfully processed all but one of the package lists, so I would have expected update-notifier to do so as well.

  I had to manually file this bug, because this error also prevents
  ubuntu-bug update-notifier from working (see attached).

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