twisted: don't encapsulate failures twice

[vincentbernat]

If a failure is already a Failure instance, don't encapsulate it into
a Failure. Keep it as is.

Fix #144

Note that I have no clue if this is actually correct. This fixes the failing test and it doesn't seem dumb to do that. I didn't investigate what changes triggered this problem. Maybe Failure was not a BaseException in the past.

[codecov]

Codecov Report

Merging #145 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #145   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          13     13           
  Lines         791    791           
  Branches      101    101           
=====================================
  Hits          791    791

Impacted Files	Coverage Δ
src/structlog/twisted.py	100% <100%> (ø)	⬆️

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[hynek]

So yeah looks fair enough, however I think we have to add a tox env for Twisted with the old behavior to ensure we don’t break anything? It should be enough if it runs just the twisted parts of the test suite.

[vincentbernat]

Do you want me to do it?

[hynek]

Yeah it would be nice if it were part of this PR. Also please remember to adapt .travis.yml.

[vincentbernat]

Done. Error seems unrelated to this PR.

[hynek]

Thanks, could you add a changelog entry too, please?

[vincentbernat]

I have modified the first commit to add a changelog entry.

[hynek]

Thanks!