Add REDUCTION_CERTIFICATE

[blegat]

Corresponds to Mosek statuses:

• https://docs.mosek.com/8.1/capi/constants.html#mosek.solsta.prim_illposed_cer
• https://docs.mosek.com/8.1/capi/constants.html#mosek.solsta.dual_illposed_cer

Closes #725

[codecov-io]

Codecov Report

Merging #734 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #734   +/-   ##
=======================================
  Coverage   94.24%   94.24%           
=======================================
  Files          55       55           
  Lines        5852     5852           
=======================================
  Hits         5515     5515           
  Misses        337      337

Impacted Files	Coverage Δ
src/attributes.jl	94.59% <ø> (ø)	⬆️

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[Peiffap]

I think you're supposed to put a hyphen in "ill-posed".

[blegat]

I think you're supposed to put a hyphen in "ill-posed".

Indeed, I used illposed as this is what is used in the Mosek doc but it seems ill-posed is the correct form.