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(axforxinrange(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.