Improve error handling for completion command

[varunkashyap]

We currently only support bash completion scripts, however as raised as part (1) of #1012 this means we output bash completion script irrespective of what the user types as the shell.

This PR makes typing bash explicit and mandatory, introduces valid args and checks for length and presence of valid args

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[codecov-io]

Codecov Report

Merging #1206 into master will decrease coverage by 0.03%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #1206      +/-   ##
==========================================
- Coverage   41.45%   41.41%   -0.04%     
==========================================
  Files          95       95              
  Lines        4229     4233       +4     
==========================================
  Hits         1753     1753              
- Misses       2305     2309       +4     
  Partials      171      171

Impacted Files	Coverage Δ
cmd/skaffold/app/cmd/completion.go	0% <0%> (ø)	⬆️

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