[feature] Make transit_model::Result generic on Error

[woshilapin]

This small PR takes inspiration from this little tip.

Note - This is not a breaking change even if we change the type transit_model::Result which is part of our API.

[datanel]

Is it in Draft for discussion or you are working on it? (label WIP)

[woshilapin]

It's a draft for discussion, that's why I didn't put the wip label.

[datanel]

but we never put wip when it's draft. anyway
It seems good to me

[woshilapin]

Another side effect is this. Before, the documentation would appear as

Now, it will appear as

And when the error type is already transit_model::Error, the documentation does not change, it is showing the same thing as before (does not show the error type when it is the default one).

StdResult felt like a new type: the reader of the documentation would need to check to understand (arguably one more click to understand the signature of the function). Now, it shows like the standard Result: the reader already knows... although, it's a little bit misleading because it looks like it is the standard Result but in fact, it is transit_model::Result. Personally, I'm fine with this because I don't see any impact of this misleading.