Bump up auxiliary coverage

[RMeli]

Start addressing #597; it would be nice to have a higher coverage (see #2410, #2453). This PR starts to bump up test coverage for auxiliary by testing raised exceptions.

Changes made in this Pull Request:

• Added tests for exceptions in auxiliary

PR Checklist

• Tests?
• Issue raised/referenced?

[codecov]

Codecov Report

Merging #2570 into develop will increase coverage by 0.09%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop    #2570      +/-   ##
===========================================
+ Coverage    90.68%   90.78%   +0.09%     
===========================================
  Files          169      169              
  Lines        22833    22832       -1     
  Branches      2940     2940              
===========================================
+ Hits         20707    20728      +21     
+ Misses        1540     1525      -15     
+ Partials       586      579       -7

Impacted Files	Coverage Δ
package/MDAnalysis/auxiliary/XVG.py	94.69% <ø> (+6.97%)	⬆️
auxiliary/base.py	91.29% <0%> (+0.96%)	⬆️
package/MDAnalysis/auxiliary/base.py	88.92% <0%> (+1.62%)	⬆️
package/MDAnalysis/auxiliary/core.py	95.65% <0%> (+26.08%)	⬆️

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[RMeli]

For some reason I can't suggest a reviewer, but I think @fiona-naughton will be the most appropriate. :)

[orbeckst]

I requested @fiona-naughton for you.

[fiona-naughton]

Looks good to me! (other than maybe a few extraneous blank lines if we care about keeping the style consistent)

[RMeli]

Thanks for the comments @fiona-naughton; the blank lines are flake8's fault! ;) I can revert them if preferred, but they should be conformant with PEP8.

[fiona-naughton]

Ah, fair enough - should be fine, then!

[orbeckst]

@fiona-naughton thanks.

In the future, can you please leave your reviews via the code reviews (click on files changed unless there's a button with your name on it when you've been requested)(and click approve/request changes to make it clearer that you reviewed the code and had an opinion? That's going to be the pattern for GSoC so I'd like to get everyone used to it.

[orbeckst]

lgtm

Thanks @fiona-naughton for the review!