Clean up failure messaging when bad CHOST

This cleans up the failure messaging when CHOST is invalid in #4177.

Before:
```
msf > use auxiliary/scanner/sip/options
msf auxiliary(options) > set CHOST 1.2.3.4
CHOST => 1.2.3.4
msf auxiliary(options) > set RHOSTS 10.4.18.0/24
RHOSTS => 10.4.18.0/24
msf auxiliary(options) > run

[*] Scanned 256 of 256 hosts (100% complete)
[-] Fatal: The source IP (CHOST) value of 1.2.3.4 was not usable
[*] Scan terminated due to one or more fatal errors
[*] Auxiliary module execution completed
```

After:

```
msf > use auxiliary/scanner/sip/options
msf auxiliary(options) > set CHOST 1.2.3.4
CHOST => 1.2.3.4
msf auxiliary(options) > set RHOSTS 10.4.18.0/24
RHOSTS => 10.4.18.0/24
msf auxiliary(options) > run

[-] Fatal: The source IP (CHOST) value of 1.2.3.4 was not usable
[-] Scan terminated due to one or more fatal errors
[*] Auxiliary module execution completed
```