Error on invalid artifact workspace

[briandealwis]

Fixes: #4491

Description

Adds a validation step to ensure that all artifact workspaces exist.

Invalid location

--- examples/getting-started/skaffold.yaml
+++ examples/getting-started/skaffold.yaml
@@ -3,6 +3,7 @@ kind: Config
 build:
   artifacts:
   - image: skaffold-example
+    context: this/path/does/not/exist
 deploy:
   kubectl:
     manifests:

$ skaffold build
invalid skaffold config: image "skaffold-example" context "this/path/does/not/exist" does not exist

---

File

diff --git examples/getting-started/skaffold.yaml examples/getting-started/skaffold.yaml
index cbf95a1a7..3cd2940ac 100644
--- examples/getting-started/skaffold.yaml
+++ examples/getting-started/skaffold.yaml
@@ -3,6 +3,7 @@ kind: Config
 build:
   artifacts:
   - image: skaffold-example
+    context: main.go
 deploy:
   kubectl:
     manifests:

$ skaffold build
invalid skaffold config: image "skaffold-example" context "main.go" is not a directory

[codecov]

Codecov Report

Merging #4492 into master will decrease coverage by 0.00%.
The diff coverage is 83.33%.

@@            Coverage Diff             @@
##           master    #4492      +/-   ##
==========================================
- Coverage   72.37%   72.36%   -0.01%     
==========================================
  Files         333      333              
  Lines       12944    12956      +12     
==========================================
+ Hits         9368     9376       +8     
- Misses       2980     2982       +2     
- Partials      596      598       +2     

Impacted Files	Coverage Δ
pkg/skaffold/schema/validation/validation.go	93.89% <83.33%> (-1.07%)	⬇️
...affold/kubernetes/portforward/kubectl_forwarder.go	60.97% <0.00%> (-2.44%)	⬇️

---

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[MarlonGamez]

This LGTM!