Put IdeResult in terms of Either, add ideError

[Gurkenglas]

Let's try this again.

[Avi-D-coder]

@Gurkenglas What is the motive for this change?
It is allot more concise.

[Gurkenglas]

You have answered your question :3

[jneira]

Thanks for the pr, i think the intent is good and i always like the prs that remove locs (the actual new type has its own advantages though).
However, as you surely already knows, we just have start a process of integration with ghcide so it is the actual main focus.
We will continue to fix bugs but we will prefer that the new code or internal refactorings go in that direction (see #1416 (comment)).