Fix file appender rules truncation now that I use a custom filebuf.

src/loggers/file_appender.cxx

@@ -57,12 +57,12 @@ std::shared_ptr<appender> file_appender::create(properties const & props) {
 }
 
 static std::ios_base::openmode mode(bool append) {
-  return append ? std::ios_base::app : 0;
+  return append ? std::ios_base::app : std::ios_base::out;
 }
 
 buffer::buffer(properties const & props) : std::filebuf() {
   if (open(props["filename"], mode(props["fileAppend"])) == nullptr) {
-    // TODO: throw
+    throw std::runtime_error("Cannot open file");
   }
 }