Give informative message for parsing errors

[timholy]

Fixes https://github.com/JuliaCI/Coverage.jl/issues/299
Closes #42

[codecov-io]

Codecov Report

Merging #43 (0d8c65f) into master (47f880e) will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #43      +/-   ##
==========================================
+ Coverage   96.41%   96.46%   +0.04%     
==========================================
  Files           4        4              
  Lines         223      226       +3     
==========================================
+ Hits          215      218       +3     
  Misses          8        8              

Impacted Files	Coverage Δ
src/CoverageTools.jl	98.31% <100.00%> (+0.04%)	⬆️

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[coveralls]

Pull Request Test Coverage Report for Build 83

• 4 of 4 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.04%) to 97.297%

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Totals
Change from base Build 75:	0.04%
Covered Lines:	180
Relevant Lines:	185

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💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 79

• 13 of 13 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.04%) to 97.297%

---

Totals
Change from base Build 75:	0.04%
Covered Lines:	180
Relevant Lines:	185

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💛 - Coveralls

[timholy]

Thanks for fixing it!

[fingolfin]

Thanks for making the PR :-) Very nice improvement