fix: Treat malformed HTTP responses as connection errors (#63)

By default, OkHTTP would throw an IOException when an illegal HTTP response is returned, which is an unexpected exception. This PR replaces that exception with java.net.ConnectException, which we catch and wrap with a PoWeb `ServerConnectionException`, which provides more useful information.

Before:

```
java.io.IOException: unexpected end of stream on http://127.0.0.1:13276/...
        at okhttp3.internal.http1.Http1ExchangeCodec.readResponseHeaders(Http1ExchangeCodec.kt:202)
        (...)
     Caused by: java.io.EOFException: \n not found: limit=0 content=…
        at okio.RealBufferedSource.readUtf8LineStrict(RealBufferedSource.kt:348)
        (...)
```

[Now](https://console.firebase.google.com/u/0/project/relaynet-gateway-android/testlab/histories/bh.fce322cdd2254629/matrices/6474453305210171425/executions/bs.a50e5565a3e74f36/testcases/5/test-cases), using relaycorp/relaynet-gateway-android@731ab77:

```
tech.relaycorp.poweb.ServerConnectionException: Failed to connect to http://127.0.0.1:13276/v1/parcels
(...)
Caused by: java.net.ConnectException: Failed to connect to /127.0.0.1:13276
(...)
Caused by: java.net.ConnectException: failed to connect to /127.0.0.1 (port 13276) from /127.0.0.1 (port 43068) after 10000ms: (...)
Caused by: android.system.ErrnoException: isConnected failed: ECONNREFUSED (Connection refused)

```

src/main/kotlin/tech/relaycorp/poweb/PoWebClient.kt

@@ -24,6 +24,7 @@ import kotlinx.coroutines.channels.ClosedReceiveChannelException
 import kotlinx.coroutines.flow.Flow
 import kotlinx.coroutines.flow.FlowCollector
 import kotlinx.coroutines.flow.flow
+import okhttp3.OkHttpClient
 import tech.relaycorp.poweb.handshake.Challenge
 import tech.relaycorp.poweb.handshake.InvalidChallengeException
 import tech.relaycorp.poweb.handshake.Response
@@ -55,7 +56,14 @@ public class PoWebClient internal constructor(
     internal val hostName: String,
     internal val port: Int,
     internal val useTls: Boolean,
-    ktorEngine: HttpClientEngine = OkHttp.create {}
+    ktorEngine: HttpClientEngine = OkHttp.create {
+        // By default, OkHTTP would throw an IOException when an illegal HTTP response is returned.
+        // Enabling retryOnConnectionFailure would treat that as a connection failure and try again,
+        // throwing a java.net.ConnectException if it still fails -- And ConnectException is a
+        // more reliable exception to handle when something like this goes wrong. See:
+        // https://github.com/relaycorp/relaynet-poweb-jvm/issues/61
+        preconfigured = OkHttpClient.Builder().retryOnConnectionFailure(true).build()
+    }
 ) : Closeable {
     internal var ktorClient = HttpClient(ktorEngine) {
         install(WebSockets)