Suggestion: Handle failures in individual update checks more gracefully

[NoiTheCat]

If one of the items that updates are checked against fails, the applet does not show anything regarding other updates. For example, as most recently implied in #146, a failed attempt to fetch widget information caused the applet to not return any updates for pacman and AUR.

My suggestion is allowing individual checks to fail without stopping the rest of the process, and then reporting the failures however possible.

[exequtic]

What was done:

• If an error occurs during one of the update checks, the process no longer stops. Instead, it continues to the next stage, and a single notification with all accumulated errors is shown at the end.
• Previously, repository and AUR update checks were performed with a single command.
  They are now separated, if AUR is unavailable, it no longer affects fetching updates from official repositories.
• More status messages were added during the update check.