Use cobra's RunE wherever possible

[imjasonh]

Notably, this makes ko exit 1 when kubectl isn't installed, instead of logging and otherwise doing nothing.

Less notably, this makes the error from ko run slightly less verbose.

Otherwise no functional changes are expected.

[codecov-commenter]

Codecov Report

Merging #343 (751f32d) into main (75ab991) will increase coverage by 0.12%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main     #343      +/-   ##
==========================================
+ Coverage   38.24%   38.37%   +0.12%     
==========================================
  Files          33       33              
  Lines        1527     1522       -5     
==========================================
  Hits          584      584              
+ Misses        849      844       -5     
  Partials       94       94              

Impacted Files	Coverage Δ
pkg/commands/apply.go	0.00% <0.00%> (ø)
pkg/commands/create.go	0.00% <0.00%> (ø)
pkg/commands/delete.go	0.00% <0.00%> (ø)
pkg/commands/publish.go	0.00% <0.00%> (ø)
pkg/commands/resolve.go	0.00% <0.00%> (ø)
pkg/commands/run.go	0.00% <0.00%> (ø)

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