stills_process: crash on unhandled command line argument

[dwpaley]

Currently if I type

$ dials.stills_process data.h5 params.phil abcd=efgh

or worse, one of:

$ dials.stills_process data.h5 params.phll
$ dials.stills_process data.h5 params.phil spotfinding.filter.d_max=10 # oops, should be spotfinder

the unhandled argument is ignored with possibly confusing consequences. Currently all unhandled arguments are assumed to be input paths. Can we validate that they are actually paths?

[codecov]

Codecov Report

Merging #2106 (76f6ceb) into main (c645ecc) will increase coverage by 0.00%.
The diff coverage is 100.00%.

❗ Current head 76f6ceb differs from pull request most recent head 7401fc5. Consider uploading reports for the commit 7401fc5 to get more accurate results

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  Files         580      580           
  Lines       65706    65708    +2     
  Branches     9260     9260           
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+ Hits        52805    52810    +5     
+ Misses      10847    10846    -1     
+ Partials     2054     2052    -2