ErrorResponse: drastically improved error messages, no more "unknown error"s (#2660)

## Description:

We've gotten several reports with error messages that aren't very
helpful, like #2390.

Turns out that whenever the API returned a non-successful response but
without an error code, we always produced _"unknown error"_.
This improves the behavior to default to showing the status code. One
example that was very obvious is setting up the SDK with an invalid API
key.

## Examples:

### `OfferingsManager.Error`:

- Before:
```
[Purchases] - ERROR: 🍎‼️ Error fetching offerings - The operation couldn’t be completed. (RevenueCat.OfferingsManager.Error error 0.)
Underlying error: The operation couldn’t be completed. (RevenueCat.NetworkError error 6.)
```

- After:
```
[Purchases] - ERROR: 🍎‼️ Error fetching offerings - The operation couldn’t be completed. (RevenueCat.OfferingsManager.Error error 0.)
Underlying error: Unknown error. API request failed with status code 401
```

### `BackendError`:
- Before:
```
[Purchases] - ERROR: 😿‼️ Unknown error.
```

- After:
```
[Purchases] - ERROR: 😿‼️ Unknown error. API request failed with status code 401
```

### Request debug logs:
- Before:
```
[Purchases] - DEBUG: ℹ️ API request failed: GET /v1/subscribers/$RCAnonymousID:988f9b85609b4a57af255750bec1fbb2/offerings: Unknown error.
```
- After:
```
[Purchases] - DEBUG: ℹ️ API request failed: GET /v1/subscribers/$RCAnonymousID:988f9b85609b4a57af255750bec1fbb2/offerings: Unknown error. API request failed with status code 401
```

### `CustomerInfoManager`:
- Before:
```
[Purchases] - WARN: ⚠️ Attempt to update CustomerInfo from network failed.
The operation couldn’t be completed. (RevenueCat.BackendError error 0.)
Underlying error: Unknown error
```

- After:
```
[Purchases] - WARN: ⚠️ Attempt to update CustomerInfo from network failed.
The operation couldn’t be completed. (RevenueCat.BackendError error 0.)
Underlying error: Unknown error. API request failed with status code 401
```

Sources/Networking/HTTPClient/HTTPClient.swift

@@ -358,11 +358,10 @@ private extension HTTPClient {
         let urlRequest = self.convert(request: request.adding(defaultHeaders: self.defaultHeaders))
 
         guard let urlRequest = urlRequest else {
-            Logger.error("Could not create request to \(request.path)")
+            let error: NetworkError = .unableToCreateRequest(request.httpRequest.path)
 
-            request.completionHandler?(
-                .failure(.unableToCreateRequest(request.httpRequest.path))
-            )
+            Logger.error(error.description)
+            request.completionHandler?(.failure(error))
             return
         }