Server exits with unhelpful error when ssl.enabled=true, but cert/key files are unspecified

[axw]

When you configure apm-server with ssl.enabled: true, but don't specify either ssl.certificate or ssl.key, then apm-server will exit with the following error message:

Exiting: open : no such file or directory

The docs say:

certificate
The path to the file containing the certificate for Server authentication. Required if apm-server.ssl.enabled is true.

key
The path to the file containing the Server certificate key. Required if apm-server.ssl.enabled is true.

We should validate these requirements during startup, and exit with a more helpful error message, suggesting what needs to be changed.

[simitt]

It might make sense to make the changes directly in libbeat, as this error message is currently the same for all the beats.

[axw]

Verified with BC2. Procedure:

• Started stack with apm-it
• Modified docker-compose.yml, adding -E apm-server.ssl.enabled=true to apm-server command
• Found a moderately more useful error message:

localtesting_7.9.0_apm-server | Exiting: Error processing configuration: certificate file not configured accessing 'apm-server.ssl' (source:'command line flag')