Add check to ensure input file was successfully opened in NNVM deploy…

… code demo (apache#4315)

docs/deploy/nnvm.md

@@ -59,9 +59,11 @@ An example in c++.
 #include <tvm/runtime/registry.h>
 #include <tvm/runtime/packed_func.h>
 
+#include <algorithm>
 #include <fstream>
 #include <iterator>
-#include <algorithm>
+#include <stdexcept>
+#include <string>
 
 int main()
 {
@@ -97,7 +99,9 @@ int main()
     int64_t in_shape[4] = {1, 3, 224, 224};
     TVMArrayAlloc(in_shape, in_ndim, dtype_code, dtype_bits, dtype_lanes, device_type, device_id, &x);
     // load image data saved in binary
-    std::ifstream data_fin("cat.bin", std::ios::binary);
+    const std::string data_filename = "cat.bin";
+    std::ifstream data_fin(data_filename, std::ios::binary);
+    if(!data_fin) throw std::runtime_error("Could not open: " + data_filename);
     data_fin.read(static_cast<char*>(x->data), 3 * 224 * 224 * 4);
 
     // get the function from the module(set input data)