fix gpu detection

[roywei]

Summary

From MXNet 1.5, the detection of number of GPUs changed from using command linenvidia-smi to MXNet C API call using CUDA.

As a result, keras-mxnet should catch both CalledProcessError and MXNetError

Related Issues

PR Overview

• This PR requires new unit tests [y/n] (make sure tests are included)
• This PR requires to update the documentation [y/n] (make sure the docs are up-to-date)
• This PR is backwards compatible [y/n]
• This PR changes the current API [y/n]

[sandeep-krishnamurthy]

When will this error be thrown?

[roywei]

When will this error be thrown?

@sandeep-krishnamurthy when GPU driver not installed or driver version and cuda version mismatch.

We used CalledProcessError to catch nvidia-smi failure before, but now the error type changed to MXNetError in MXNet 1.5