check if file exists in mnist_loader (#1127)

doc/examples/mnist_mlp/mnist_loader.cpp

@@ -80,6 +80,12 @@ void MNistLoader::open()
         throw std::logic_error("Loader already open");
     }
     m_file = fopen(m_filename.c_str(), "rb");
+    if (m_file == nullptr)
+    {
+        throw std::runtime_error(std::string("File") + m_filename +
+                                 "couldn't be opened. Make sure the file "
+                                 "exists in the current directory");
+    }
     read_header();
     fgetpos(m_file, &m_data_pos);
 }