Per #1103, we should update linkerd inject to emit warnings if the config file supplied to it cannot be injected. Some of the reasons why the config can't be injected are as follows:

• Modify inject to warn when file is un-injectable #1603 It's malformed
• Modify inject to warn when file is un-injectable #1603 It uses Host networking
• Inject warns on UDP ports #1617 It uses UDP, not TCP
• Introduce inject check for known sidecars #1619 It contains init containers for other known sidecars
  • linkerd
  • istio
  • contour
• Modify inject to warn when file is un-injectable #1603 It doesn't contain any injectable resources