Ensure and improve Seravo Plugin exception safety

[Moppa5]

Description
Seravo Plugin has had many bugs related to assuming and blindly executing PHP code. For example, developing on local environment some fopen() etc. have caused many issues when the resource hasn't existed at all. As a part of the cleanup and Up-To-Date we should investigate carefully and ensure that Seravo Plugin handles these situations properly. Some parts can be checked with PHP return values and some with try catch statements.

This issue can include any PHP warnings & buggy code as well as they usually fall into the exception category.

[JoosuaKoskinen]

I'm currently working on the new postbox system and one of the many new features is exception handling. If exception is thrown during the data fetch step of postbox rendering, user is shown proper error message and given instructions on opening an issue.

We should see if there's more we can do though.