Changelog and version updates for v0.78.0
Make cli/exec_io only raise DXCLIError and do so for --input parsing errors
Fix docstring typo.
Fix error handling for setting executable in dx add/update stage
Improve dx describe workflow and better error handling

- IO specs displayed when describing by name
- Inaccessible stages rendered as such in describe, list stages
- dx run refuses to run a workflow with inaccessible stages
Remove extraneous TODO comment
Re-enable tests that depend on now-released features
Temporarily skip tests that require unreleased features