Improve error message for command vmcreds2pdf

The error message Error: VM(s) not foundcould occur for several reasons, including that the corresponding VMs are not started. The message was modified to include potential causes of errors to guide the user toward potential solutions.

requested review from @florent.gluck

assigned to @florent.gluck

mentioned in commit 9af3f72e

merged

mentioned in commit 587e2187

mentioned in commit 9742dbaa

mentioned in commit a0cdfa7a