Fix inconsistent behavior of existing session when using -d compared to --files option: raise an AssertionError instead of just a warning

[neurorepro]

This is described in #683

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.06 :tada:

Comparison is base (e3c53b8) 81.94% compared to head (a7d4af1) 82.00%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #682 +/- ## ========================================== + Coverage 81.94% 82.00% +0.06% ========================================== Files 41 41 Lines 4132 4130 -2 ========================================== + Hits 3386 3387 +1 + Misses 746 743 -3 ``` | [Impacted Files](https://app.codecov.io/gh/nipy/heudiconv/pull/682?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=nipy) | Coverage Δ | | |---|---|---| | [heudiconv/parser.py](https://app.codecov.io/gh/nipy/heudiconv/pull/682?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=nipy#diff-aGV1ZGljb252L3BhcnNlci5weQ==) | `96.87% <100.00%> (+2.99%)` | :arrow_up: |

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[yarikoptic]

change of behavior so adding minor label instead of just a patch

[github-actions[bot]]

:rocket: PR was released in v1.0.0 :rocket: