Add MonadError.rethrow

[SystemFw]

This PR seeks to add rethrow to MonadError, i.e. the inverse of attempt.
The law for it is currently failing on IorT.

[kailuowang]

still have scala style errors

[error] /home/travis/build/typelevel/cats/core/src/main/scala/cats/MonadError.scala:47:4: Whitespace at end of line
[error] /home/travis/build/typelevel/cats/core/src/main/scala/cats/MonadError.scala:52:4: Whitespace at end of line
[error] /home/travis/build/typelevel/cats/core/src/main/scala/cats/MonadError.scala:56:4: Whitespace at end of line
[error] /home/travis/build/typelevel/cats/core/src/main/scala/cats/MonadError.scala:61:5: Whitespace at end of line

and also need to add mima exceptions to here

Update actually let me do that for you.

[SystemFw]

@kailuowang you are too kind :)

[johnynek]

👍

[codecov-io]

Codecov Report

Merging #2061 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2061      +/-   ##
==========================================
+ Coverage   94.66%   94.66%   +<.01%     
==========================================
  Files         318      318              
  Lines        5380     5383       +3     
  Branches      112      207      +95     
==========================================
+ Hits         5093     5096       +3     
  Misses        287      287

Impacted Files	Coverage Δ
core/src/main/scala/cats/data/Ior.scala	100% <ø> (ø)	⬆️
laws/src/main/scala/cats/laws/MonadErrorLaws.scala	100% <100%> (ø)	⬆️
core/src/main/scala/cats/MonadError.scala	100% <100%> (ø)	⬆️
core/src/main/scala/cats/data/IorT.scala	97.39% <100%> (-0.03%)	⬇️
core/src/main/scala/cats/syntax/monadError.scala	100% <100%> (ø)	⬆️
...n/scala/cats/laws/discipline/MonadErrorTests.scala	100% <100%> (ø)	⬆️

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[kailuowang]

We already have 2 sign-offs, but since I made a couple lines of changes here. Maybe @LukaJCB can give it another look?

[mpilquist]

Is it weird to call this rethrow when E could be any type (i.e. not a Throwable)?

[SystemFw]

It is a bit weird, and we had a brief discussion on gitter cause I was not sure about the original name.
I started with absolve, then materialize/unmaterialize were floating around, in the end I've settled with rethrow. reraise is more consistent I guess, but it's fairly ugly as well

[mpilquist]

:) I have no objections

[kailuowang]

Hmm I do like reraise better, not a strong preference though

[SystemFw]

I don't have a strong preference either tbh

[kailuowang]

do you want to change to reraise? or I can make another PR against your branch?

[SystemFw]

It's past two so I won't be able to do anything until tomorrow
Go ahead if you have time though :)

[kailuowang]

never mind about the rename, I noticed that the word "rethrow" is already used numerous time in MonadError's doc.

[LukaJCB]

LGTM :)