Make error handling in XMLDecoder simpler & safer

[regexident]

There were way too many functions returning throw -> _? in XMLDecoder, which should only ever return throw -> _ (turning the nil case into an error of its own).

👷🏻‍♀️ I also managed to get rid of quite a bunch of force-unwraps.
Especially the ones eventually leading to crashes in #38 and #34. 💥

[codecov]

Codecov Report

Merging #41 into master will decrease coverage by 0.04%.
The diff coverage is 56.6%.

@@            Coverage Diff             @@
##           master      #41      +/-   ##
==========================================
- Coverage   51.46%   51.41%   -0.05%     
==========================================
  Files          29       29              
  Lines        1399     1412      +13     
==========================================
+ Hits          720      726       +6     
- Misses        679      686       +7

Impacted Files	Coverage Δ
Sources/XMLCoder/Decoder/XMLDecoder.swift	39.81% <53.19%> (+0.92%)	⬆️
Sources/XMLCoder/Decoder/XMLDecodingStorage.swift	72.72% <83.33%> (-9.1%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 4cad2df...b752e0d. Read the comment docs.