Ensure S3Exception has message

Also add missing toString implementation.

src/main/java/org/gaul/s3proxy/S3Exception.java

@@ -44,7 +44,7 @@ public final class S3Exception extends Exception {
 
     S3Exception(S3ErrorCode error, String message, Throwable cause,
                 Map<String, String> elements) {
-        super(message, cause);
+        super(requireNonNull(message), cause);
         this.error = requireNonNull(error);
         this.elements = ImmutableMap.copyOf(elements);
     }
@@ -56,4 +56,9 @@ public final class S3Exception extends Exception {
     Map<String, String> getElements() {
         return elements;
     }
+
+    @Override
+    public String toString() {
+        return error + " " + elements;
+    }
 }

src/main/java/org/gaul/s3proxy/S3ProxyHandler.java

@@ -1079,7 +1079,8 @@ public class S3ProxyHandler {
         }
         if (!created) {
             throw new S3Exception(S3ErrorCode.BUCKET_ALREADY_OWNED_BY_YOU,
-                    null, null, ImmutableMap.of("BucketName", containerName));
+                    S3ErrorCode.BUCKET_ALREADY_OWNED_BY_YOU.getMessage(),
+                    null, ImmutableMap.of("BucketName", containerName));
         }
 
         response.addHeader(HttpHeaders.LOCATION, "/" + containerName);