Improve error message when skaffold config not found

[nkubala]

before:

➜  skaffold dev
[skaffold.yaml] not found. You might need to run `skaffold init`

after:

➜  skaffold dev
skaffold config file skaffold.yaml not found - check your current working directory, or try running `skaffold init`

[codecov]

Codecov Report

Merging #4679 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #4679   +/-   ##
=======================================
  Coverage   73.20%   73.21%           
=======================================
  Files         339      339           
  Lines       13322    13323    +1     
=======================================
+ Hits         9753     9754    +1     
  Misses       2958     2958           
  Partials      611      611           

Impacted Files	Coverage Δ
cmd/skaffold/app/cmd/runner.go	78.94% <100.00%> (ø)
pkg/skaffold/deploy/kustomize.go	77.84% <0.00%> (+0.13%)	⬆️

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