Improve error message for command vmcreds2pdf

src/client/cmdVM/vmCreds2pdf.go

@@ -50,7 +50,7 @@ func (cmd *Creds2pdf)Run(args []string) int {
     }
 
     if len(vms) == 0 {
-        u.PrintlnErr("Error: VM(s) not found.")
+        u.PrintlnErr("Error: VM(s) not found (possible causes: no VM by that ID/regex, VM(s) not started, no right to list requested VM(s)).")
         return 1
     }