revamp exception handling

[Herringway]

This is pretty big, but a large chunk of it is unittests. This makes exceptions thrown by the library much more consistent as well as allowing them to provide additional context. A later PR will enable the test suite to take advantage of this new capability as well.

[codecov]

Codecov Report

Attention: Patch coverage is 93.93140% with 23 lines in your changes are missing coverage. Please review.

Project coverage is 93.73%. Comparing base (8b18bb0) to head (bd74554).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #329      +/-   ##
==========================================
- Coverage   93.74%   93.73%   -0.02%     
==========================================
  Files          29       29              
  Lines        4570     4770     +200     
==========================================
+ Hits         4284     4471     +187     
- Misses        286      299      +13     

Files	Coverage Δ
source/dyaml/composer.d	97.87% <100.00%> (+1.12%)	⬆️
source/dyaml/loader.d	95.60% <100.00%> (+1.23%)	⬆️
source/dyaml/node.d	91.07% <ø> (-0.78%)	⬇️
source/dyaml/parser.d	98.85% <100.00%> (+0.08%)	⬆️
source/dyaml/representer.d	99.48% <ø> (ø)
source/dyaml/scanner.d	100.00% <100.00%> (+0.34%)	⬆️
source/dyaml/test/reader.d	75.00% <ø> (ø)
source/dyaml/reader.d	96.60% <80.00%> (-0.86%)	⬇️
source/dyaml/constructor.d	94.05% <93.00%> (+1.68%)	⬆️
source/dyaml/exception.d	68.29% <67.50%> (-22.62%)	⬇️

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