Rephrase "Exceptions" to "Errors" and add orion::hash to root docs

[brycx]

Fixes #60.

[codecov]

Codecov Report

Merging #62 into master will decrease coverage by 0.19%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master      #62     +/-   ##
=========================================
- Coverage   96.33%   96.13%   -0.2%     
=========================================
  Files          46       46             
  Lines        6624     6624             
=========================================
- Hits         6381     6368     -13     
- Misses        243      256     +13

Impacted Files	Coverage Δ
src/pwhash.rs	97.01% <ø> (ø)	⬆️
src/aead.rs	98.82% <ø> (ø)	⬆️
src/hazardous/aead/chacha20poly1305.rs	89.35% <ø> (ø)	⬆️
src/hazardous/aead/xchacha20poly1305.rs	85.58% <ø> (ø)	⬆️
src/hazardous/mac/hmac.rs	96.68% <ø> (ø)	⬆️
src/hazardous/hash/blake2b.rs	95% <ø> (-2.86%)	⬇️
src/hazardous/xof/cshake.rs	97.69% <ø> (-0.39%)	⬇️
src/kdf.rs	96.05% <ø> (ø)	⬆️
src/hazardous/stream/xchacha20.rs	84.71% <ø> (ø)	⬆️
src/hazardous/hash/sha512.rs	98.51% <ø> (ø)	⬆️
... and 9 more

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