Some Exception-related fixes

[Herringway]

Just some cleanup related to exception handling.

• some Exceptions used int for line numbers instead of size_t
• most of Scanner isn't nothrow/nogc so avoiding throwing Exceptions directly wasn't saving us anything

[codecov[bot]]

Codecov Report

Merging #241 into master will decrease coverage by <.01%. The diff coverage is 95.45%.

@@            Coverage Diff             @@
##           master     #241      +/-   ##
==========================================
- Coverage   94.89%   94.89%   -0.01%     
==========================================
  Files          28       28              
  Lines        4254     4191      -63     
==========================================
- Hits         4037     3977      -60     
+ Misses        217      214       -3

Impacted Files	Coverage Δ
source/dyaml/node.d	92.45% <100%> (ø)	:arrow_up:
source/dyaml/reader.d	97.94% <100%> (ø)	:arrow_up:
source/dyaml/constructor.d	92.01% <100%> (ø)	:arrow_up:
source/dyaml/scanner.d	99.32% <95.23%> (+0.7%)	:arrow_up:
source/dyaml/exception.d	86.66% <0%> (-13.34%)	:arrow_down:

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[Herringway]

Some of the logic in Scanner was painfully convoluted because of that error handling... am really glad to delete it here