Provide info when no Backend was loaded.

src/main/java/ch/psi/daq/queryrest/controller/QueryRestController.java

@@ -95,11 +95,14 @@ public class QueryRestController {
 
    @PostConstruct
    public void afterPropertiesSet() {
+      List<Exception> exceptions = new ArrayList<>();
+      
       try {
          QueryProcessor queryProcessor =
                appContext.getBean(QueryConfig.BEAN_NAME_CASSANDRA_QUERY_PROCESSOR, QueryProcessor.class);
          queryProcessors.put(queryProcessor.getBackend(), queryProcessor);
       } catch (Exception e) {
+         exceptions.add(e);
          LOGGER.warn("");
          LOGGER.warn("##########");
          LOGGER.warn("Could not load query processor for cassandra.");
@@ -112,6 +115,7 @@ public class QueryRestController {
                appContext.getBean(QueryConfig.BEAN_NAME_ARCHIVER_APPLIANCE_QUERY_PROCESSOR, QueryProcessor.class);
          queryProcessors.put(queryProcessor.getBackend(), queryProcessor);
       } catch (Exception e) {
+         exceptions.add(e);
          LOGGER.warn("");
          LOGGER.warn("##########");
          LOGGER.warn("Could not load query processor for archiverappliance.");
@@ -119,6 +123,15 @@ public class QueryRestController {
          LOGGER.warn("");
       }
       
+      if(queryProcessors.isEmpty()){
+         LOGGER.error("No query processor could be loaded! Exceptions were: ");
+         for (Exception exception : exceptions) {
+            LOGGER.error("", exception);
+         }
+         
+         throw new RuntimeException("No Backends available!");
+      }
+
       channelNameCache =
             new ChannelNameCache(queryProcessors, appContext.getBean(CassandraConfig.BEAN_NAME_READ_TIMEOUT,
                   Integer.class).longValue());