Nested error reporting not walking clause chain

[marcotc]

The implementation of nested error reporting in #1472 had a bug in which it did not correctly visit the full #cause error chain in the reported exception, only the first cause was reported.

This PR correctly navigate the whole cause chain and reports such information.

[ivoanjo]

Change looks good (was this my derp? ahaha oops I guess I optimized it a bit extra by removing part of the functionality), but the failures in both JRuby configurations seem to be deterministic.

Could you look into what's up with them?

[marcotc]

@ivoanjo is was a simple JRuby test assertion failure, but it turned into a crash in CI 😮

[codecov-commenter]

Codecov Report

Merging #1535 (17db1f5) into master (b05fddf) will increase coverage by 0.00%.
The diff coverage is 92.85%.

@@           Coverage Diff           @@
##           master    #1535   +/-   ##
=======================================
  Coverage   98.23%   98.23%           
=======================================
  Files         877      877           
  Lines       42261    42267    +6     
=======================================
+ Hits        41517    41523    +6     
  Misses        744      744           

Impacted Files	Coverage Δ
spec/ddtrace/error_spec.rb	94.26% <91.66%> (+0.24%)	⬆️
lib/ddtrace/error.rb	97.72% <100.00%> (+0.05%)	⬆️

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