Improve error message when secure connection cannot be established when adding MySQL with pmm-admin.
- When secure connection cannot be established (due to IP/host verification failure). Instead of
"Connection check failed: x509: cannot validate certificate for 127.0.0.1 because it doesn't contain any IP SANs." show easy to understand message.
- Provide link to a guide/doc that explains why user sees that error and how to fix it.