When passing a revision to `rbt diff` or `rbt post` in a Jujutsu repository, a typo in the change ID would produce an unhelpful error: CRITICAL: Unexpected error executing the command: jj log -r ... The actual error from jj was swallowed. A revset that matched no changes, such as `none()`, crashed with "list index out of range". `parse_revision_spec` now raises `InvalidRevisionSpecError` for these cases. The message includes the error from jj, such as: CRITICAL: Invalid revision "kkkkqqqq": Revision \`kkkkqqqq\` doesn't exist This also adds special handling for the "zzzzzzzz" root commit, which would attempt to look up the parent and then fail later with a confusing fork point error. Testing Done: - Ran unit tests. - Ran `rbt diff` with a typoed change ID, an empty revset, a revset with a syntax error, the root commit ID, and a valid change.