TASK: Handle non-integer error codes in throwabe FileStorage

[kdambekalns]

This will no longer swallow certain error codes but instead emit them in the message.

Checklist

• Code follows the PSR-2 coding style
• Tests have been created, run and adjusted as needed
• The PR is created against the lowest maintained branch
• Reviewer - PR Title is brief but complete and starts with FEATURE|TASK|BUGFIX
• Reviewer - The first section explains the change briefly for change-logs
• Reviewer - Breaking Changes are marked with !!! and have upgrade-instructions

[markusguenther]

That looks reasonable and logical. :)