Fix lost filename in err msg

In some circumstances, FileExceptions are constructed empty, then have a
filename assigned to them, but the error message in these scenarios is
left as the default "unknown" one, which is sometimes shown to users.
This change fixes that case to be consistent with instances that are
constructed with the filename.

src/Base/Exception.h

@@ -262,6 +262,7 @@ protected:
   // necessary   for what() legacy behaviour as it returns a buffer that
   // can not be of a temporary object to be destroyed at end of what()
   std::string _sErrMsgAndFileName;
+  void setFileName(const char * sFileName=0);
 };
 
 /**