ci: Fix lint issues, makefile target

[mway]

Noticed that there were a handful of lint issues, all errcheck things. This PR:

• Adds underscores to clarify error discarding
• Updates the make golangci-lint target to use ./..., as e.g. there are no exp/*.go files

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.70%. Comparing base (4425037) to head (4890655).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1443   +/-   ##
=======================================
  Coverage   98.69%   98.70%           
=======================================
  Files          53       53           
  Lines        2996     3014   +18     
=======================================
+ Hits         2957     2975   +18     
  Misses         31       31           
  Partials        8        8           

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