Minor UX fix: sky cancel should not print stacktraces. (#1385)

* Minor UX fix: `sky cancel` should not print stacktraces.

* Wording fix.

* exit 1

sky/cli.py

@@ -1582,6 +1582,9 @@ def cancel(cluster: str, all: bool, jobs: List[int]):  # pylint: disable=redefin
         core.cancel(cluster, all, jobs)
     except ValueError as e:
         raise click.UsageError(str(e))
+    except (exceptions.NotSupportedError, exceptions.ClusterNotUpError) as e:
+        click.echo(str(e))
+        sys.exit(1)
 
 
 @cli.command(cls=_DocumentedCodeCommand)

sky/core.py

@@ -320,7 +320,7 @@ def _check_cluster_available(cluster_name: str,
         with ux_utils.print_exception_no_traceback():
             raise exceptions.ClusterNotUpError(
                 f'{colorama.Fore.YELLOW}Cluster {cluster_name!r} does not '
-                f'exist.{colorama.Style.RESET_ALL} skipped.')
+                f'exist; skipped.{colorama.Style.RESET_ALL}')
     backend = backend_utils.get_backend_from_handle(handle)
     if isinstance(backend, backends.LocalDockerBackend):
         # LocalDockerBackend does not support job queues