ENH: Add check for 0 covariance in studentization

[bashtage]

Ensure covariance is positive and meaningfully large

[pep8speaks]

Hello @bashtage! Thanks for updating this PR. We checked the lines you've touched for PEP 8 issues, and found:

There are currently no PEP 8 issues detected in this Pull Request. Cheers! 🍻

Comment last updated at 2019-11-05 15:12:56 UTC

[coveralls]

Coverage increased (+0.0006%) to 99.597% when pulling 61f7d47 on studentization-error into 6434d57 on master.

[codecov]

Codecov Report

Merging #322 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #322      +/-   ##
==========================================
+ Coverage   99.41%   99.41%   +<.01%     
==========================================
  Files          46       46              
  Lines       10493    10508      +15     
  Branches      907      908       +1     
==========================================
+ Hits        10432    10447      +15     
  Misses         18       18              
  Partials       43       43

Impacted Files	Coverage Δ
arch/tests/bootstrap/test_bootstrap.py	100% <100%> (ø)	⬆️
arch/utility/exceptions.py	100% <100%> (ø)	⬆️
arch/bootstrap/base.py	96.66% <100%> (+0.04%)	⬆️

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