errors.LibTmuxErrorCode

A union rather than string so a comparison against a name this package does not have is a type error rather than a branch that never runs.

Each class declares this type rather than its own literal because the hierarchy is three deep in places — NoMatchError extends ObjectNotFoundError — and a literal on the parent makes the child's declaration an illegal override. instanceof is the narrowing tool; this is the one that survives a bundler and a serialization round trip.