Fix 404 handling for invalid blob references in snippets

What does this MR do and why?

Contributes to #564937 (closed)

Problem

When invalid paths are passed to snippet blob actions, ExtractsRef::RefExtractor::InvalidPathError exceptions are raised instead of returning a proper 404 response to users.

Solution

Handle an exception and return a 404 error when provided reference is incorrect.

References

Unhandled `ExtractsRef::RefExtractor::InvalidPa... (#564937 - closed)

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Related to #564937 (closed)