`land` should exit gracefully when there are non-submitted changes, now it crashes

[ZolotukhinM]

Repro:

1. make change zzz
2. stack-pr export
3. make change ppp
4. stack-pr view:

VIEW
Stack:
   * 3344c73c (no PR, 'ZolotukhinM/stack/133' -> 'ZolotukhinM/stack/132'): ppp
   * a7883442 (#139, 'ZolotukhinM/stack/132' -> 'main'): zzz

5. stack-pr land

LAND
Traceback (most recent call last):
  File "/Users/mvz/.local/bin/stack-pr", line 7, in <module>
    sys.exit(main())
             ~~~~^^
  File "/Users/mvz/.local/pipx/venvs/stack-pr/lib/python3.13/site-packages/stack_pr/cli.py", line 1659, in main
    command_land(common_args)
    ~~~~~~~~~~~~^^^^^^^^^^^^^
  File "/Users/mvz/.local/pipx/venvs/stack-pr/lib/python3.13/site-packages/stack_pr/cli.py", line 1241, in command_land
    set_base_branches(st, args.target)
    ~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^
  File "/Users/mvz/.local/pipx/venvs/stack-pr/lib/python3.13/site-packages/stack_pr/cli.py", line 483, in set_base_branches
    e.base, prev_branch = prev_branch, e.head
                                       ^^^^^^
  File "/Users/mvz/.local/pipx/venvs/stack-pr/lib/python3.13/site-packages/stack_pr/cli.py", line 300, in head
    raise ValueError("head is not set")
ValueError: head is not set