Split SyntaxError::UnclosedTag into UnclosedTag and UnclosedAttributeValue

[Mingun]

For more precise error reporting.

Closes #926.

Currently error is reported at start of a tag instead of start of an attribute value.

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 66.66667% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 54.55%. Comparing base (d1acdb5) to head (229d84c).
⚠️ Report is 24 commits behind head on master.

Files with missing lines	Patch %	Lines
src/errors.rs	0.00%	2 Missing ⚠️
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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #932      +/-   ##
==========================================
- Coverage   58.08%   54.55%   -3.53%     
==========================================
  Files          42       44       +2     
  Lines       15513    16914    +1401     
==========================================
+ Hits         9011     9228     +217     
- Misses       6502     7686    +1184     

Flag	Coverage Δ
unittests	54.55% <66.66%> (-3.53%)	⬇️

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[dralley]

IMO an enum type would be preferable to bool

[Mingun]

I thinked about UncloseSingleQuotedAttributeValue and UncloseDoubleQuotedAttributeValue instead of one variant. It seems to me that no need to introduce UnclosedAttributeValue(enum QuoteType), because QuoteType seems will be used only here, and errors usually anyway will be just passed above.

So I can change as described, agree?