A wrong configuration of CasC yaml kills Jenkins startup

### Your checklist for this issue

🚨 Please review the [guidelines for contributing](../blob/master/docs/CONTRIBUTING.md) to this repository.

- [x] Jenkins version
2.60.1, 2.164.1,2.165

- [x] Plugin version
2.20-snapshot, 2.19

- [x] OS
Linux

### Description

While testing the CasC plugin I've found that if the yaml is wrong, Jenkins stop restarting, leaving you with an unoperational instance.

Steps to reproduce:

Cre

- ate a yaml with ssh credentials
- Import this yaml
- Remove credentials and ssh-credentials
- Restart Jenkins

**Expected:**
Some settings are established, others not.

**Current behavior:**
Jenkins doesn't start.

Also filed in Jira: https://issues.jenkins-ci.org/browse/JENKINS-57862