Reduce usage of BoolError for manual code

[sdroege]

We have quite some manual code where we currently use BoolError although there are multiple, proper error cases. For example ObjectExt::set_property() or all the signals related functions.

I would suggest to replace these all with proper error types.

@GuillaumeGomez @EPashkin opinions?

[GuillaumeGomez]

Same as in the PR discussion: I agree on using more accurate types instead.

[EPashkin]

I also agree that concrete type is better

[elmarco]

@sdroege the idea is to replace BoolError with more specific errors for the various domain/cases? Similar to value::GetError, for other types?

[sdroege]

Yes, where it makes sense at least.

[elmarco]

What about public functions that return BoolError? Should we just switch to a different type? Should BoolError wrap the specific type?

[sdroege]

I don't know. It requires proper design work for each of the cases.