Use ConnectError.unavailable when client is offline (#328)

Resolves #325.

When turning the device's wifi off, the following was observed with
URLSession:

Before:

```
▿ Optional<ConnectError>
  ▿ some : ConnectError
    - code : Connect.Code.unknown
    ▿ message : Optional<String>
      - some : "The Internet connection appears to be offline."
    ▿ exception : Optional<Error>
      - some : Error Domain=NSURLErrorDomain Code=-1009 "The Internet connection appears to be offline." 
```

Based on the discussion in the issue linked above, this PR updates the
code to be `.unavailable`:

```
▿ Optional<ConnectError>
  ▿ some : ConnectError
    - code : Connect.Code.unavailable
    ▿ message : Optional<String>
      - some : "The Internet connection appears to be offline."
    ▿ exception : Optional<Error>
      - some : Error Domain=NSURLErrorDomain Code=-1009 "The Internet connection appears to be offline." 
```

The NIO client has also been updated to return the `.unavailable` status
code:

```
▿ Optional<ConnectError>
  ▿ some : ConnectError
    - code : Connect.Code.unavailable
    ▿ message : Optional<String>
      - some : "client is not connected"
    - exception : nil
```

---------

Signed-off-by: Michael Rebello <me@michaelrebello.com>

Author: rebello95

Libraries/Connect/Public/Implementation/Clients/URLSessionHTTPClient.swift

@@ -203,10 +203,18 @@ extension Code {
             return .canceled
         case URLError.badURL.rawValue:
             return .invalidArgument
-        case URLError.timedOut.rawValue:
+        case URLError.timedOut.rawValue, NSURLErrorTimedOut:
             return .deadlineExceeded
         case URLError.unsupportedURL.rawValue:
             return .unimplemented
+        case NSURLErrorCannotConnectToHost,
+            NSURLErrorCannotFindHost,
+            NSURLErrorDataNotAllowed,
+            NSURLErrorInternationalRoamingOff,
+            NSURLErrorNetworkConnectionLost,
+            NSURLErrorNotConnectedToInternet,
+            NSURLErrorSecureConnectionFailed:
+            return .unavailable
         case ...100: // URLSession can return errors in this range
             return .unknown
         default:

Libraries/ConnectNIO/Public/NIOHTTPClient.swift

@@ -41,10 +41,6 @@ open class NIOHTTPClient: Connect.HTTPClientInterface, @unchecked Sendable {
         case connected(channel: NIOCore.Channel, multiplexer: NIOHTTP2.HTTP2StreamMultiplexer)
     }
 
-    private enum Error: Swift.Error {
-        case disconnected
-    }
-
     /// Designated initializer for the client.
     ///
     /// - parameter host: Target host (e.g., `https://connectrpc.com`).
@@ -137,11 +133,11 @@ open class NIOHTTPClient: Connect.HTTPClientInterface, @unchecked Sendable {
                 )
             } else {
                 onResponse(.init(
-                    code: .unknown,
+                    code: .unavailable,
                     headers: [:],
                     message: nil,
                     trailers: [:],
-                    error: Error.disconnected,
+                    error: ConnectError.disconnected(),
                     tracingInfo: nil
                 ))
             }
@@ -165,7 +161,7 @@ open class NIOHTTPClient: Connect.HTTPClientInterface, @unchecked Sendable {
                     for: request.url, on: eventLoop, using: multiplexer, with: handler
                 )
             } else {
-                responseCallbacks.receiveClose(.unknown, [:], Error.disconnected)
+                responseCallbacks.receiveClose(.unavailable, [:], ConnectError.disconnected())
             }
         }
         return .init(
@@ -268,3 +264,9 @@ open class NIOHTTPClient: Connect.HTTPClientInterface, @unchecked Sendable {
         try? self.loopGroup.syncShutdownGracefully()
     }
 }
+
+private extension ConnectError {
+    static func disconnected() -> Self {
+        return .init(code: .unavailable, message: "client is not connected")
+    }
+}