Bug: Improve error handling when not using encrypted key (without password)

[kairoaraujo]

What happened?

RSTUF requires an encrypted key for Root keys and encrypted keys for online keys when using PEM format.
Right now, we don't handle this well in the code (as of in the docs).

Please enter [yellow]path[/] to encrypted private key '[green]<securesystemslib.signer._key.SSlibKey object at 0x75ed13f2e1d0>[/]'/home/user/Desktop/repository-service-tuf/keys/keypair.pem

Please enter password to encrypted private key 'ali1': 
Traceback (most recent call last):
  File "/home/user/.local/bin/rstuf", line 8, in <module>
    sys.exit(rstuf())
             ^^^^^^^
  File "/home/user/.local/pipx/venvs/repository-service-tuf/lib/python3.11/site-packages/rich_click/rich_command.py", line 367, in __call__
    return super().__call__(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/user/.local/pipx/venvs/repository-service-tuf/lib/python3.11/site-packages/click/core.py", line 1161, in __call__
    return self.main(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/user/.local/pipx/venvs/repository-service-tuf/lib/python3.11/site-packages/rich_click/rich_command.py", line 152, in main
    rv = self.invoke(ctx)
         ^^^^^^^^^^^^^^^^
  File "/home/user/.local/pipx/venvs/repository-service-tuf/lib/python3.11/site-packages/click/core.py", line 1697, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/user/.local/pipx/venvs/repository-service-tuf/lib/python3.11/site-packages/click/core.py", line 1697, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/user/.local/pipx/venvs/repository-service-tuf/lib/python3.11/site-packages/click/core.py", line 1443, in invoke
    return ctx.invoke(self.callback, **ctx.params)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/user/.local/pipx/venvs/repository-service-tuf/lib/python3.11/site-packages/click/core.py", line 788, in invoke
    return __callback(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/user/.local/pipx/venvs/repository-service-tuf/lib/python3.11/site-packages/click/decorators.py", line 33, in new_func
    return f(get_current_context(), *args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/user/.local/pipx/venvs/repository-service-tuf/lib/python3.11/site-packages/repository_service_tuf/cli/admin/ceremony.py", line 137, in ceremony
    _add_root_signatures_prompt(root_md, None)
  File "/home/user/.local/pipx/venvs/repository-service-tuf/lib/python3.11/site-packages/repository_service_tuf/cli/admin/helpers.py", line 633, in _add_root_signatures_prompt
    _add_signature_prompt(root_md, key_choices[choice])
  File "/home/user/.local/pipx/venvs/repository-service-tuf/lib/python3.11/site-packages/repository_service_tuf/cli/admin/helpers.py", line 589, in _add_signature_prompt
    signer = _load_signer_from_file_prompt(key)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/user/.local/pipx/venvs/repository-service-tuf/lib/python3.11/site-packages/repository_service_tuf/cli/admin/helpers.py", line 254, in _load_signer_from_file_prompt
    private_key = load_pem_private_key(private_pem, password.encode())
                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
TypeError: Password was given but private key is not encrypted.

```code

### What steps did you take?

Se the parent issue for it.

### What behavior did you expect?

_No response_

### Relevant log output

```shell

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