fix: Return error when YAML submission is invalid

[simster7]

When submitting a file with invalid YAML the CLI output is not helpful:

$ argo submit invalid.yaml
INFO[2021-11-02T12:25:12.398Z] No Workflow found in given files

This PR prints the relevant YAML parsing error

$ argo submit invalid.yaml
ERRO[2021-11-02T12:25:12.397Z] yaml file at index 0 is not valid: error converting YAML to JSON: yaml: line 12: did not find expected key
INFO[2021-11-02T12:25:12.398Z] No Workflow found in given files

Signed-off-by: Simon Behar simbeh7@gmail.com

Don't bother creating a PR until you've done this:

• Run make pre-commit -B to fix codegen, lint, and commit message problems.

Create your PR as a draft.

• Your PR needs to pass the required checks before it can be approved. If the check is not required (e.g. E2E tests) it
  does not need to pass.
• Once required tests have passed, you can make it "Ready for review".
• Say how how you tested your changes. If you changed the UI, attach screenshots.

Tips:

• If changes were requested, and you've made them, then dismiss the review to get it looked at again.
• Add you organization to USERS.md if you like.
• You can ask for help!