Add mechanism to print exception in Kafka test thread

Adds a mechanism (which was added to KafkaEventStreamDecoder
incorrectly) to print the exception to help flush out our flakey tests.

See commit 92455e43 - which is the same changes in
KafkaEventStreamDecoderTest for details

Framework/LiveData/test/KafkaHistoStreamDecoderTest.h

@@ -105,38 +105,49 @@ private:
   void startCapturing(Mantid::LiveData::KafkaHistoStreamDecoder &decoder,
                       uint8_t maxIterations) {
     // Register callback to know when a whole loop as been iterated through
+    m_maxIterations = maxIterations;
     m_niterations = 0;
-    auto callback = [this, maxIterations]() {
-      this->iterationCallback(maxIterations);
-    };
-    decoder.registerIterationEndCb(callback);
-    decoder.registerErrorCb(callback);
+    decoder.registerIterationEndCb([this]() { this->iterationCallback(); });
+
+    decoder.registerErrorCb([this, &decoder]() { errCallback(decoder); });
     TS_ASSERT_THROWS_NOTHING(decoder.startCapture());
     continueCapturing(decoder, maxIterations);
   }
 
-  void iterationCallback(uint8_t maxIterations) {
+  void iterationCallback() {
     std::unique_lock<std::mutex> lock(this->m_callbackMutex);
     this->m_niterations++;
-    if (this->m_niterations == maxIterations) {
+    if (this->m_niterations == m_maxIterations) {
       lock.unlock();
       this->m_callbackCondition.notify_one();
     }
   }
 
-  void continueCapturing(Mantid::LiveData::KafkaHistoStreamDecoder &decoder,
+  void errCallback(KafkaHistoStreamDecoder &decoder) {
+    try {
+      // Get the stored exception by calling extract data again
+      decoder.extractData();
+    } catch (std::exception &e) {
+      // We could try to port the exception from this child thread to the main
+      // thread, or just print it here which is significantly easier
+      std::cerr << "Exception: " << e.what() << "\n";
+      // Always keep incrementing so we don't deadlock
+      iterationCallback();
+    }
+  }
+
+  void continueCapturing(KafkaHistoStreamDecoder &decoder,
                          uint8_t maxIterations) {
+    m_maxIterations = maxIterations;
+
     // Re-register callback with the (potentially) new value of maxIterations
-    auto callback = [this, maxIterations]() {
-      this->iterationCallback(maxIterations);
-    };
-    decoder.registerIterationEndCb(callback);
-    decoder.registerErrorCb(callback);
+    decoder.registerIterationEndCb([this]() { this->iterationCallback(); });
+    decoder.registerErrorCb([this, &decoder]() { errCallback(decoder); });
+
     {
       std::unique_lock<std::mutex> lk(m_callbackMutex);
-      this->m_callbackCondition.wait(lk, [this, maxIterations]() {
-        return this->m_niterations == maxIterations;
-      });
+      this->m_callbackCondition.wait(
+          lk, [this]() { return this->m_niterations == m_maxIterations; });
     }
   }
 
@@ -183,4 +194,5 @@ private:
   std::mutex m_callbackMutex;
   std::condition_variable m_callbackCondition;
   uint8_t m_niterations = 0;
+  std::atomic<uint8_t> m_maxIterations = 0;
 };