Skaffold should error on an invalid context directory

[briandealwis]

Expected behavior

Noticed by Rune FS on StackOverflow: Skaffold does not raise an error on startup if an artifact has an invalid context directory, and instead produces a difficult to decipher error.

Actual behavior

--- 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
Generating tags...
 - skaffold-example -> skaffold-example:latest
Some taggers failed. Rerun with -vdebug for errors.
Checking cache...
 - skaffold-example: Error checking cache.
getting hash for artifact "skaffold-example": getting dependencies for "skaffold-example": file pattern [main.go] must match at least one file

Try without the build-cache:

$ skaffold  build --cache-artifacts=false
Generating tags...
 - skaffold-example -> skaffold-example:latest
Some taggers failed. Rerun with -vdebug for errors.
Found [minikube] context, using local docker daemon.
Building [skaffold-example]...
Sending build context to Docker daemon 

couldn't build "skaffold-example": docker build: error during connect: Post "https://192.168.192.145:2376/v1.40/build?buildargs=null&cachefrom=null&cgroupparent=&cpuperiod=0&cpuquota=0&cpusetcpus=&cpusetmems=&cpushares=0&dockerfile=Dockerfile&forcerm=1&labels=null&memory=0&memswap=0&networkmode=&rm=0&shmsize=0&t=skaffold-example%3Alatest&target=&ulimits=null&version=": creating docker context: getting relative tar paths: file pattern [main.go] must match at least one file