fix(log): handle invalid test/series IDs without uncaught ValueError

[SAY-5]

Code review checklist:

• Code is generally sensical and well commented
• Variable/function names all telegraph their purpose and contents
• Functions/classes have useful doc strings
• Function arguments are typed
• Added/modified unit tests to cover changes.
• New features have documentation added to the docs.
• New features and backwards compatibility breaks are noted in the RELEASE.md

Fixes #1064.

When pav log <subcmd> <id> is passed an id that is neither a valid TestID nor a valid SeriesID, LogCommand.run falls through to SeriesID(args.id), whose constructor raises an uncaught ValueError and crashes the CLI.

This guards the conversion with SeriesID.is_valid_id and prints a readable error before returning 1, matching how the other "load failed" paths in this command already behave. Adds a regression test.