raise MXNetError(py_str(_LIB.MXGetLastError()))

### my computer don't have GPU. I modify:
```
detector = RetinaFace('./model/R50', -1, gpuid, 'net3')
```

### when run Test.py. This shows error:

```
raise MXNetError(py_str(_LIB.MXGetLastError()))
mxnet.base.MXNetError: [09:04:55] /tmp/build/80754af9/libmxnet_1564766659613/work/3rdparty/dmlc-core/src/io/local_filesys.cc:209: Check failed: allow_null: LocalFileSystem::Open "./model/R50--001.params": No such file or directory
Stack trace:
[bt] (0) /home/tu/anaconda3/lib/python3.7/site-packages/mxnet/libmxnet.so(dmlc::io::LocalFileSystem::Open(dmlc::io::URI const&, char const*, bool)+0x5a9) [0x7f006ebb08a9]
[bt] (1) /home/tu/anaconda3/lib/python3.7/site-packages/mxnet/libmxnet.so(dmlc::Stream::Create(char const*, char const*, bool)+0x241) [0x7f006eb80251]
[bt] (2) /home/tu/anaconda3/lib/python3.7/site-packages/mxnet/libmxnet.so(MXNDArrayLoad+0xa9) [0x7f006c7ceb89]
[bt] (3) /home/tu/anaconda3/lib/python3.7/lib-dynload/../../libffi.so.6(ffi_call_unix64+0x4c) [0x7f007c671ec0]
[bt] (4) /home/tu/anaconda3/lib/python3.7/lib-dynload/../../libffi.so.6(ffi_call+0x22d) [0x7f007c67187d]
[bt] (5) /home/tu/anaconda3/lib/python3.7/lib-dynload/_ctypes.cpython-37m-x86_64-linux-gnu.so(_ctypes_callproc+0x2ce) [0x7f007c887f7e]
[bt] (6) /home/tu/anaconda3/lib/python3.7/lib-dynload/_ctypes.cpython-37m-x86_64-linux-gnu.so(+0x139b4) [0x7f007c8889b4]
[bt] (7) python(_PyObject_FastCallKeywords+0x49b) [0x55e1848e9d2b]
[bt] (8) python(_PyEval_EvalFrameDefault+0x537e) [0x55e1849457ae]
```