Error type updates

[sagebind]

Update isahc::Error to support taking new variants in a backwards-compatible way, as well as hold more error sources when an error is caused by another.

Note that this is the first 1.0 breaking change to land on master. If 0.9 needs a patch fix in the future, we'll have to make a 0.9.x branch for it.

Fixes #182.

[codecov]

Codecov Report

Merging #258 (fed6133) into master (e7284e3) will decrease coverage by 0.24%.
The diff coverage is 47.53%.

@@            Coverage Diff             @@
##           master     #258      +/-   ##
==========================================
- Coverage   72.11%   71.86%   -0.25%     
==========================================
  Files          49       49              
  Lines        2460     2513      +53     
==========================================
+ Hits         1774     1806      +32     
- Misses        686      707      +21     

Impacted Files	Coverage Δ
src/body.rs	75.38% <ø> (ø)
src/config/mod.rs	56.19% <ø> (ø)
src/interceptor/mod.rs	0.00% <ø> (ø)
src/lib.rs	75.00% <ø> (ø)
src/error.rs	40.57% <40.87%> (+11.66%)	⬆️
src/client.rs	69.66% <55.55%> (-0.04%)	⬇️
src/agent.rs	68.58% <100.00%> (ø)
src/handler.rs	71.68% <100.00%> (ø)
src/interceptor/context.rs	75.00% <100.00%> (ø)
src/interceptor/obj.rs	100.00% <100.00%> (ø)
... and 5 more

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