fix NameError handling in expression code

[shazarivf]

Fixes #129

@newville could you take a look please?

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[newville]

@shazarivf Thanks! Looking at this some more, I might add a few more changes in the error handling too.

[shazarivf]

@newville Thanks! Can a release be made with this fix anytime soon?

[shazarivf]

@newville could you tell me when a release can be made with this change? It's a blocker for us.

[newville]

@shazarivf 1.0.2 has been tagged and pushed to PyPI. I don't usually consider each bug fix to require a release. FWIW, asking for a release of open-source software twice in two days is probably not going to be a winning strategy very often.

[shazarivf]

@newville thanks for making an exception. Yeah I get it.