Fix potential memory leaks

[taminob]

While looking through the source code, I found a few potential memory leaks.

Most of them were caused by early returns (e.g. because of an exception) without freeing a resource first.
To prevent those and make the code more robust, I added a scope_guard which will ensure via RAII that the resources are freed.
Additionally, I added them in a few places where GErrors are in use and where I think there might be a risk in the future.

Instead of this custom scope_guard it could also be considered to use a third-party library but I think it's easier to provide this simple functionality in util instead of integrating yet another dependency.

I tried to group the different kinds of fixes I did in different commits, I'll squash them in the end to make history more readable.
Also, I made the fixes as small as possible to avoid regressions or break something by accident.

[taminob]

Sorry that I forgot to auto-format the commits. Should be fixed now.

I noticed that the CI doesn't seem to check the hpp files which also had a formatting inconsistency in the previous commits (see #2589).

[Alexays]

LGTM thanks!