Fix confusing error message when adding signature parameters that conflict with existing arguments

[mauvilsa]

Before submitting

• [x] Did you read the contributing guideline?
• [n/a] Did you update the documentation? (readme and public docstrings)
• [x] Did you write unit tests such that there is 100% coverage on related code? (required for bug fixes and new features)
• [x] Did you verify that new and existing tests pass locally?
• [x] Did you make sure that all changes preserve backward compatibility?
• [x] Did you update the CHANGELOG? (not for typos, docs, test updates, or minor internal changes/refactors)

[sonarcloud[bot]]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

No Coverage information
0.0% Duplication

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (0e37aca) 100.00% compared to head (209d643) 100.00%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #420 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 21 21 Lines 5934 5938 +4 ========================================= + Hits 5934 5938 +4 ``` | [Flag](https://app.codecov.io/gh/omni-us/jsonargparse/pull/420/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=omni-us) | Coverage Δ | | |---|---|---| | [py3.10](https://app.codecov.io/gh/omni-us/jsonargparse/pull/420/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=omni-us) | `88.27% <100.00%> (+<0.01%)` | :arrow_up: | | [py3.10_all](https://app.codecov.io/gh/omni-us/jsonargparse/pull/420/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=omni-us) | `98.72% <100.00%> (+<0.01%)` | :arrow_up: | | 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[py3.12_types](https://app.codecov.io/gh/omni-us/jsonargparse/pull/420/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=omni-us) | `98.72% <100.00%> (+<0.01%)` | :arrow_up: | | [py3.7](https://app.codecov.io/gh/omni-us/jsonargparse/pull/420/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=omni-us) | `88.85% <100.00%> (+<0.01%)` | :arrow_up: | | [py3.7_all](https://app.codecov.io/gh/omni-us/jsonargparse/pull/420/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=omni-us) | `99.49% <100.00%> (+<0.01%)` | :arrow_up: | | [py3.7_types](https://app.codecov.io/gh/omni-us/jsonargparse/pull/420/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=omni-us) | `99.47% <100.00%> (+<0.01%)` | :arrow_up: | | [py3.8](https://app.codecov.io/gh/omni-us/jsonargparse/pull/420/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=omni-us) | `89.00% <100.00%> (+<0.01%)` | :arrow_up: | | [py3.8_all](https://app.codecov.io/gh/omni-us/jsonargparse/pull/420/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=omni-us) | `99.51% <100.00%> (+<0.01%)` | :arrow_up: | | [py3.8_types](https://app.codecov.io/gh/omni-us/jsonargparse/pull/420/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=omni-us) | `99.49% <100.00%> (+<0.01%)` | :arrow_up: | | [py3.9](https://app.codecov.io/gh/omni-us/jsonargparse/pull/420/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=omni-us) | `88.85% <100.00%> (+<0.01%)` | :arrow_up: | | 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