HTTP Error Handling Problem

@Takumi2008 is already working on this.

Since Jun 9, 2023.

### Proposal
When a requested value is not found in the local or hot cache within `groupcache`, the system performs a consistent hash on the key to determine which instance owns the requested value. If it determines that the value is owned by the local instance, it calls the `GetterFunc()` to retrieve the value. If `GetterFunc()` returns an error during the local call, `groupcache` propagates that error to the caller of `group.Get()`, allowing the caller to handle the error appropriately.

However, if `groupcache` determines that the value is owned by a remote instance, it makes an HTTP call to that instance which invokes `GetterFunc()` on the remote instance. If the `GetterFunc()` returns an error during the remote call, `groupcache` returns an `http.StatusInternalServerError` error to the caller. When this happens the calling instance of, `groupcache` logs this error (if a logger is set) and proceeds to fall back to calling `GetterFunc()` locally in an attempt to retrieve the value.

This situation is suboptimal for a few reasons
1. In the case where the `GetterFunc` returns a `not found` error during a remote HTTP call, it makes no sense for the calling instance to fall back to calling `GetterFunc` locally which will likely result in also returning `not found`. Especially if the `GetterFunc` is retrieving the value from a common database.
2. The actual error is lost when making remote calls.
3. If any remote call has an error, then a local call will be made as a result. This could result in the duplicate `GetterFunc` calls exacerbating the underlying problem which caused the error.

##### Solution
`groupcache` should add an `ErrNotFound` error to the library. This error is provided such that implementors of `GetterFunc` can return this error to indicate the call failed to find the request the value. Remote calls via HTTP will reflect this error by returning `http.StatusNotFound`

All other errors will be returned via HTTP will be returned with `http.StatusServiceUnavailable` instead of the current `http.StatusInternalServerError`. This differentiates between an internal error (Something is wrong with `groupcache` internals) and an error returned by the `GetterFunc`.

The `groupcache` instance which made the HTTP call will look for these codes and avoid making a local `GetterFunc` call, and instead will propagate the error back to the caller with either `ErrNotFound` or `ErrRemoteCall` which contains the string value of the error returned by `GetterFunc`.