Change warning when Internet is not available

[sunkup]

Please delete this paragraph and other repeating text (like the examples) after reading and before submitting the PR.

Purpose

Currently the warning says "No validated Internet connectivity. Synchronization may not run.", however it's not "maybe", but it will certainly not work. So we should use a better warning text.

Short description

Add new warning text in english and german

• Automatic synchronization not active (no verified Internet connection)
• Automatische Synchronisierung nicht aktiv (keine überprüfte Internetverbindung)

Checklist

• The PR has a proper title, description and label.
• I have self-reviewed the PR.
• I have added documentation to complex functions and functions that can be used by other modules.
• I have added reasonable tests or consciously decided to not add tests.

[rfc2822]

The German translation won't be used. We have to commit the English strings.xml first, and then Transifex will either

• fetch it automatically within one day,
• or we can upload it with tx push -s.

As soon as the English string is on Transifex, we can translate it on Transifex and it will be taken over when we download them the next time (usually before a release).