JAVA-1140: Implemented BulkWriteError.toString

Author: jyemin

src/main/com/mongodb/BulkWriteError.java

@@ -115,4 +115,14 @@ public int hashCode() {
         result = 31 * result + details.hashCode();
         return result;
     }
+
+    @Override
+    public String toString() {
+        return "BulkWriteError{"
+               + "index=" + index
+               + ", code=" + code
+               + ", message='" + message + '\''
+               + ", details=" + details
+               + '}';
+    }
 }