Convert methods on TimeDelta to return Result

[pitdicker]

I kept TimeDelta::{checked_add, checked_sub} like they are, returning an Option. That seems in line with checked_add on integers.

We are now at a point where clean-ups are possible. The last commit gets rid of the expect(ok!(/* ... */)) that developed while parts of the API returned Result and parts returned Option.

Once this PR and #1537 land we can get rid of the ok! and try_opt! macros.

[codecov]

Codecov Report

Attention: Patch coverage is 94.36620% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 93.98%. Comparing base (21ee9b7) to head (bb85a51).

Files	Patch %	Lines
src/time_delta.rs	96.55%	2 Missing ⚠️
src/lib.rs	66.66%	1 Missing ⚠️
src/naive/datetime/mod.rs	83.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##            0.5.x    #1538      +/-   ##
==========================================
+ Coverage   93.96%   93.98%   +0.02%     
==========================================
  Files          37       37              
  Lines       16662    16656       -6     
==========================================
- Hits        15656    15654       -2     
+ Misses       1006     1002       -4     

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