Don't run the plugin loader if the dir is missing

Description

Skips the plugin loader if the configured plugin directory does not exist. This is a common case if the core is being run for the first time. A warning is logged instead.

Additions

• Added a check to see if the directory exists. If it does not, a warning is logged and loading plugins is aborted.

Test and Review

Resolves #255 (closed)

To be filled in by the reviewers

• All of the methods are commented to expectation

• The methods are tested to satisfaction

• There are no unnecessary files present in the MR

• The continuous integration has no problems with the MR

• The MR is filled in as requested (including labels, milestones, and reviewers)

• The documentation is up-to-date

• All nullable parameters are marked as such