py: Fix errors are suppressed in generator comprehensions

[corona10]

Sequence.Iterate does not handle exceptions.
Iterate() should return error except StopIteration.

Fixes: #32

[corona10]

@ncw
I'd like to add a unit test for this.

list(ax for x in range(10))

Can you recommend the proper place?

[codecov-io]

Codecov Report

Merging #38 into master will decrease coverage by 0.01%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master      #38      +/-   ##
==========================================
- Coverage   65.25%   65.24%   -0.02%     
==========================================
  Files          56       56              
  Lines       10148    10150       +2     
==========================================
  Hits         6622     6622              
- Misses       3064     3066       +2     
  Partials      462      462

Impacted Files	Coverage Δ
py/sequence.go	20.43% <0%> (-0.45%)	⬇️

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[corona10]

@ncw PTAL
The coverage of unit test does not relevant with this PR

[ncw]

It looks like you've found a good place for the test :-)

I put a few little changes inline.

The coverage of unit test does not relevant with this PR

I would have thought it would be pretty well covered.

[corona10]

@ncw
Thanks for the review. I've updated the PR.
PTAL

[ncw]

That looks great thanks :-)

I'll merge it now.

It might be worth reveiwing other calls to py.Next as that fix looks like it was left over from a refactor when it used to return a boolean rather than an error.