KConfigIniBackend: Make diagnostics about escape sequences more readable

Instead of this:

    kf.config.core: "KConfigIni: In file scrcpy.desktop, line 8: " "Invalid escape sequence \"\\\"\"."

We'll be getting this:

    kf.config.core: KConfigIni: In file scrcpy.desktop, line 8: Invalid escape sequence: «\"»

Which is still our bug to fix, but at least now it's clear which one.

CCBUG: 471533