Add possibility of nested eager load builder to throw

[Frizlab]

This PR adds the possibility of throwing inside the block that adds nested eager loaders. I need this because I add eager loading as needed depending on fields (user input), recursively, and I don’t know before it’s too late whether my fields are valid.

As the with functions that are modified are marked as rethrows, this should have no impact on existing code and should not require a major version bump.

[codecov-commenter]

Codecov Report

Merging #574 (b169408) into main (cee27b8) will not change coverage. The diff coverage is 0.00%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #574 +/- ## ======================================= Coverage 47.63% 47.63% ======================================= Files 106 106 Lines 4702 4702 ======================================= Hits 2240 2240 Misses 2462 2462 ``` | [Files Changed](https://app.codecov.io/gh/vapor/fluent-kit/pull/574?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=vapor) | Coverage Δ | | |---|---|---| | [...uentKit/Query/Builder/QueryBuilder+EagerLoad.swift](https://app.codecov.io/gh/vapor/fluent-kit/pull/574?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=vapor#diff-U291cmNlcy9GbHVlbnRLaXQvUXVlcnkvQnVpbGRlci9RdWVyeUJ1aWxkZXIrRWFnZXJMb2FkLnN3aWZ0) | `0.00% <0.00%> (ø)` | |

[penny-for-vapor[bot]]

These changes are now available in 1.43.2