Add config option to fail pipeline upload on parsing warnings as well as errors

[nic-littlepay]

Description

Attempts to provide fixe for issue #3303

Context

There's slightly more context in the issue but broadly we wanted to be able to configure the agent to fail pipeline if it finds parse warnings in pipeline upload.

Changes

• Adds a new config bool flag to pipeline upload command.
• If that flag is set true, returns error on parse warnings.

Testing

• Tests have run locally (with go test ./...). Buildkite employees may check this if the pipeline has run automatically.
• Code is formatted (with go fmt ./...)