Error line pallete bug fix

[CMGreenspon]

Explicitly counts number of primary series as the length of the series object no longer seems to track that

[codecov-commenter]

Codecov Report

Patch coverage: 66.66% and project coverage change: +0.23 :tada:

Comparison is base (837514f) 26.29% compared to head (d1f1b12) 26.52%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #524 +/- ## ========================================== + Coverage 26.29% 26.52% +0.23% ========================================== Files 20 20 Lines 1240 1248 +8 ========================================== + Hits 326 331 +5 - Misses 914 917 +3 ``` | [Impacted Files](https://codecov.io/gh/JuliaPlots/StatsPlots.jl/pull/524?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaPlots) | Coverage Δ | | |---|---|---| | [src/errorline.jl](https://codecov.io/gh/JuliaPlots/StatsPlots.jl/pull/524?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaPlots#diff-c3JjL2Vycm9ybGluZS5qbA==) | `35.43% <66.66%> (+1.26%)` | :arrow_up: | ... and [2 files with indirect coverage changes](https://codecov.io/gh/JuliaPlots/StatsPlots.jl/pull/524/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaPlots)

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[CMGreenspon]

Closes #523

[BeastyBlacksmith]

Can you add a test that checks that you get the correct color for the example in #523 ?

[CMGreenspon]

I can add the check but I have a bunch of deadlines before I go on holiday for a week so it will have to wait until the week of the 20th.