Provide useful message when validation fails on checksum retrieval

src/main/java/org/commonjava/service/promote/core/ContentDigester.java

@@ -20,6 +20,7 @@
 import org.commonjava.service.promote.model.StoreKey;
 import org.commonjava.service.promote.util.ContentDigest;
 import org.commonjava.service.promote.util.ResponseHelper;
+import org.commonjava.service.promote.validate.PromotionValidationException;
 import org.eclipse.microprofile.rest.client.inject.RestClient;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
@@ -88,6 +89,12 @@ public String digest(StoreKey key, String path, ContentDigest digest) throws Exc
                 logger.debug("Retrieve failed, {}:{}, code: {}", key, path, resp.getStatus());
             }
         }
+        catch ( Exception e )
+        {
+            logger.debug("Retrieve and calculate checksum failed, {}:{}, Exception: {}", key, path, e.getMessage());
+            throw new PromotionValidationException(
+                "Retrieve and calculate checksum failed for " + key + ":" + path, e);
+        }
 
         return null;
     }

src/main/java/org/commonjava/service/promote/validate/PromotionValidationException.java

@@ -34,4 +34,14 @@ public PromotionValidationException(String s, Exception e, String ruleName) {
     public PromotionValidationException(String s) {
         super(s);
     }
+
+    @Override
+    public String toString() {
+        String baseMessage = super.toString();
+        Throwable cause = getCause();
+        if (cause != null && cause.getMessage() != null) {
+            return baseMessage + " [Cause: " + cause.getMessage() + "]";
+        }
+        return baseMessage;
+    }
 }