Improved error message when enum conversion fails.

[alphaleonis]

Added list of valid values to exception message when enum value conversion fails.

Example: Value 'verbosd' could not be converted to 'Verbosity?'. Accepted values are: Verbose, Normal, Minimal, Quiet

I confirm that the pull-request:

• [x] Follows the contribution guidelines
• [x] Is based on my own work
• [x] Is in compliance with my employer