Incorrect TLS uses too much CPU

If filing a bug please include the following:

### Nomad version

```
nomad v0.6.3
```

### Operating system and Environment details

### Issue

### Reproduction steps

1. Give Nomad wrong certificates
2. Watch CPU burn

### Nomad Server logs (if appropriate)

### Nomad Client logs (if appropriate)

### Job file (if appropriate)

Start by reproducing the issue on Nomad v0.6.3 using incorrect certificates and observing CPU usage. Trace the TLS failure path and verify that invalid certificates no longer cause sustained CPU burn.

Written by the indexing model from the issue text.