Geo: Minor refactors to verification

What does this MR do?

Refactors, each in their own commit (see commit details for more info. Please ask me for clarification, e.g. if it's not clear why it does not change behavior):

• Remove requirement for local? method
• Always do a hard check for file existence
• Remove needs_checksum? overrides
• Move needs_checksum? into should_primary_verify?
• Raise if not checksummable, instead of returning nil
• Extract blob verification specific logic

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.