Add log message in pdf plugin when poppler not available

[cydanil]

Description

Provide a log message rather than a printed traceback.

How Has This Been Tested?

This was tested on a setup where libpoppler is not installed.

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.