add: custom message for plugin errors

[kodaline]

Description

A custom warning message is shown in case an error occurs during plugin operations.
Printing stack trace with the error.

relates to issue #602

Type of change

• 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)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas

[pieroit]

Thanks @kodaline !
Moved the method plugin_specific_error_message inside the Plugin class for easier usage