`expose_value=false` causes crash when auto-completing.

[bakhtiary]

setting expose_value=false for an argument causes crash when auto-completing.

To replicate:

# example.py
import click

@click.command
@click.argument('a', expose_value=False)
def main():
    pass

main()

$ _EXAMPLE_PY_COMPLETE=zsh_complete COMP_CWORD=1 COMP_WORDS="BUG " python example.py BUG

you should get something like the following

  File "/home/amir/projects/click/bug", line 13, in <module>
    gogo()
  File "/home/amir/projects/click/src/click/core.py", line 1130, in __call__
    return self.main(*args, **kwargs)
  File "/home/amir/projects/click/src/click/core.py", line 1050, in main
    self._main_shell_completion(extra, prog_name, complete_var)
  File "/home/amir/projects/click/src/click/core.py", line 1125, in _main_shell_completion
    rv = shell_complete(self, ctx_args, prog_name, complete_var, instruction)
  File "/home/amir/projects/click/src/click/shell_completion.py", line 49, in shell_complete
    echo(comp.complete())
  File "/home/amir/projects/click/src/click/shell_completion.py", line 291, in complete
    completions = self.get_completions(args, incomplete)
  File "/home/amir/projects/click/src/click/shell_completion.py", line 272, in get_completions
    obj, incomplete = _resolve_incomplete(ctx, args, incomplete)
  File "/home/amir/projects/click/src/click/shell_completion.py", line 575, in _resolve_incomplete
    if _is_incomplete_argument(ctx, param):
  File "/home/amir/projects/click/src/click/shell_completion.py", line 439, in _is_incomplete_argument
    value = ctx.params[param.name]
KeyError: 'a'

I think the correct behavior is that nothing should happen or some thing should be printed to stdout. but it shouldn't crash

Environment:

• Python version: 3.10
• Click version: main

[jcrotts]

Changing this line to ctx.params.get(param.name) prints out the helptext for the command instead of the error, maybe preferable to changing expose_value.