Improve “upstream is not available” error to better understand what happened (#482)

* exclude Mono.empty() and write FullCause to response

* add test
Lower matcher type (#72)

src/main/kotlin/io/emeraldpay/dshackle/quorum/QuorumRequestReader.kt

@@ -229,7 +229,7 @@ class QuorumRequestReader(
     private fun noResponse(method: String, q: CallQuorum): Mono<Result> {
         return apiControl.upstreamsMatchesResponse()?.run {
             tracer.currentSpan()?.tag(SPAN_NO_RESPONSE_MESSAGE, getFullCause())
-            val cause = getCause(method) ?: return Mono.empty()
+            val cause = getCause(method) ?: return Mono.error(RpcException(1, "No response for method $method", getFullCause()))
             if (cause.shouldReturnNull) {
                 Mono.just(
                     Result(Global.nullValue, null, 1, null, null),

src/main/kotlin/io/emeraldpay/dshackle/upstream/UpstreamsMatchesResponse.kt

@@ -27,9 +27,9 @@ class UpstreamsMatchesResponse {
         }
     }
 
-    fun getFullCause(): String? =
+    fun getFullCause() =
         if (responses.isEmpty()) {
-            null
+            "Response is empty"
         } else {
             responses
                 .joinToString("; ") { "${it.upstreamId} - ${it.matchesResponse.getCause()}" }