Update FilteredEntityRef/Mut getters to return Result instead of Option

## What problem does this solve or what need does it fill?

Currently `FilteredEntityRef` look like this:
```
/// Gets access to the component of type `T` for the current entity.
/// Returns `None` if the entity does not have a component of type `T`.
#[inline]
pub fn get(&self) -> Option<&'_ T>;
```

The docstring is incorrect, you get a `None` if the entity does not have a component of type `T` OR if the FilteredEntityRef does not have the access to read `T`.

The return value of `Option` makes it impossible to distinguish between the two.

I have multiple times been stuck debugging while my entity does not have a given component T only to realize an hour later that my `FilteredEntityRef` does not have the correct access!!

## What solution would you like?

Change the methods to return a Result, where the Errors could be an enum with:
- InsufficientAccess
- ComponentNotFound