UNOMI-621 : add error logs when events are invalid (#467)

diff --git a/rest/src/main/java/org/apache/unomi/rest/deserializers/ContextRequestDeserializer.java b/rest/src/main/java/org/apache/unomi/rest/deserializers/ContextRequestDeserializer.java
index c14ec89..261a0fa 100644
--- a/rest/src/main/java/org/apache/unomi/rest/deserializers/ContextRequestDeserializer.java
+++ b/rest/src/main/java/org/apache/unomi/rest/deserializers/ContextRequestDeserializer.java
@@ -29,6 +29,8 @@
 import org.apache.unomi.api.services.PersonalizationService;
 import org.apache.unomi.rest.exception.InvalidRequestException;
 import org.apache.unomi.schema.api.SchemaService;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
 
 import java.io.IOException;
 import java.util.ArrayList;
@@ -40,6 +42,8 @@
  */
 public class ContextRequestDeserializer extends StdDeserializer<ContextRequest> {
 
+    private static final Logger logger = LoggerFactory.getLogger(ContextRequestDeserializer.class);
+
     private final SchemaService schemaService;
 
     public ContextRequestDeserializer(SchemaService schemaRegistry) {
@@ -88,6 +92,8 @@
             for (JsonNode event : events) {
                 if (schemaService.isEventValid(event.toString(), event.get("eventType").textValue())) {
                     filteredEvents.add(jsonParser.getCodec().treeToValue(event, Event.class));
+                } else {
+                    logger.error("An event was rejected - switch to DEBUG log level for more information");
                 }
             }
             cr.setEvents(filteredEvents);
diff --git a/rest/src/main/java/org/apache/unomi/rest/deserializers/EventsCollectorRequestDeserializer.java b/rest/src/main/java/org/apache/unomi/rest/deserializers/EventsCollectorRequestDeserializer.java
index 1bb272a..2114205 100644
--- a/rest/src/main/java/org/apache/unomi/rest/deserializers/EventsCollectorRequestDeserializer.java
+++ b/rest/src/main/java/org/apache/unomi/rest/deserializers/EventsCollectorRequestDeserializer.java
@@ -26,6 +26,8 @@
 import org.apache.unomi.api.EventsCollectorRequest;
 import org.apache.unomi.rest.exception.InvalidRequestException;
 import org.apache.unomi.schema.api.SchemaService;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
 
 import java.io.IOException;
 import java.util.ArrayList;
@@ -36,6 +38,8 @@
  */
 public class EventsCollectorRequestDeserializer extends StdDeserializer<EventsCollectorRequest> {
 
+    private static final Logger logger = LoggerFactory.getLogger(EventsCollectorRequestDeserializer.class);
+
     private final SchemaService schemaService;
 
     public EventsCollectorRequestDeserializer(SchemaService schemaRegistry) {
@@ -61,6 +65,8 @@
             for (JsonNode event : eventsNode) {
                 if (schemaService.isEventValid(event.toString(), event.get("eventType").textValue())) {
                     filteredEvents.add(jsonParser.getCodec().treeToValue(event, Event.class));
+                } else {
+                    logger.error("An event was rejected - switch to DEBUG log level for more information");
                 }
             }
         }