Improves custom email verification SMTP error logging

Feature context

The custom email feature for Service Desk allows customers to use their own email address for sending and receiving emails. Customers set up email forwarding to the project-specific Service Desk address and pass their SMTP credentials, so we can send emails on their behalf.

Setting things up involves a verification step, where we try to send an email using the SMTP credentials to the custom email.

What does this MR do and why?

Solves Custom email: Catch all SMTP errors and log err... (#499862)

With this MR we catch all SMTP errors when sending the verification email. Additionally we also log the concrete error class and message so we have further insights into what exactly went wrong.

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backend only change

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