EventTraceMgr: Move fclose() to destructor

Coverity complains about a missing fclose() in a non-existing
destructor. Also sprinkle in a strerror() call for fopen() to
provide a bit of a hint what might have gone wrong.

src/EventTrace.h

@@ -441,6 +441,8 @@ class EventTraceMgr {
 public:
     EventTraceMgr(const std::string& trace_file);
 
+    ~EventTraceMgr();
+
     // Generates the trace upon exit.
     void Generate();