linkerd only routes TCP data, but `linkerd inject` does not warn when it
injects into pods with ports set to `protocol: UDP`.
Modify `linkerd inject` to warn when injected into a pod with
`protocol: UDP`. The Linkerd sidecar will still be injected, but the
stderr output will include a warning.
Also add stderr checking on all inject unit tests.
Part of #1516.
Signed-off-by: Andrew Seigner <siggy@buoyant.io>
Andrew Seigner