Simplify and unify various returns

[tmthrgd]

This is cosmetic but makes the library a little simpler and more uniform.

[codecov-io]

Codecov Report

Merging #893 into master will increase coverage by 0.04%.
The diff coverage is 48.48%.

@@            Coverage Diff             @@
##           master     #893      +/-   ##
==========================================
+ Coverage   56.37%   56.42%   +0.04%     
==========================================
  Files          41       41              
  Lines       10032    10009      -23     
==========================================
- Hits         5656     5648       -8     
+ Misses       3355     3339      -16     
- Partials     1021     1022       +1

Impacted Files	Coverage Δ
tlsa.go	0% <0%> (ø)	⬆️
xfr.go	0% <0%> (ø)	⬆️
smimea.go	0% <0%> (ø)	⬆️
labels.go	93.33% <100%> (-0.07%)	⬇️
server.go	65.8% <100%> (-0.17%)	⬇️
msg.go	76.88% <42.85%> (-0.51%)	⬇️
client.go	63.86% <57.14%> (+2.35%)	⬆️
privaterr.go	68.85% <60%> (+0.59%)	⬆️
tsig.go	41.23% <66.66%> (-0.28%)	⬇️

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