[1.8.x] fix for MX_ERROR_MSG namespace

[samskalicky]

Description

When used out side of the mxnet::ext namespace the MX_ERROR_MSG macro causes errors like:

/home/ubuntu/NeoMXNet/3rdparty/mxnet/include/mxnet/lib_api.h:245:22: error: ‘MXerrorMsgs’ has not been declared
 #define MX_ERROR_MSG MXerrorMsgs::get()->add(__FILE__, __LINE__)
                      ^

The fix is to change:
https://github.com/apache/incubator-mxnet/blob/636b6e2a7482a7c242a7f5bbc5e694243e5ceae3/include/mxnet/lib_api.h#L245
to:

#define MX_ERROR_MSG mxnet::ext::MXerrorMsgs::get()->add(__FILE__, __LINE__)

So that any usage outside of the namespace will get the scope.

[mxnet-bot]

Hey @samskalicky , Thanks for submitting the PR
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[mseth10]

LGTM