In the following case, 

• Jenkins multibranch pipeline job is configured to connect to SCM (git) using an SSH key
• Jenkins master has HTTP(S) and SSH connectivity to SCM (git)
• Swarm slave agent has HTTP(S) connectivity, but not SSH connectivity to SCM (git)

The following happens:

• Job initiates from master; Jenkinsfile is read,
• Hand off to swarm agent occurs. Checkout SCM step fails (very little detail; job continues) 
• The job fails further down the line with an error due to not picking up SCM

Expected:

• An output message showing that the agent was not able to checkout SCM.
• (The job continuing is ok as you don't always need the SCM checkout files.)