WIP: add exception for unkown manifest output format

[ctb]

A minor fix to add error message for unknown output format. Noticed during #3801

[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 88.12%. Comparing base (01652ae) to head (8666d6d).

Files with missing lines	Patch %	Lines
src/sourmash/manifest.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           latest    #3804      +/-   ##
==========================================
- Coverage   88.12%   88.12%   -0.01%     
==========================================
  Files         137      137              
  Lines       22597    22598       +1     
  Branches     2299     2299              
==========================================
  Hits        19914    19914              
- Misses       2368     2369       +1     
  Partials      315      315              

Flag	Coverage Δ
hypothesis-py	25.26% <0.00%> (-0.01%)	⬇️
python	92.56% <0.00%> (-0.01%)	⬇️
rust	81.63% <ø> (ø)

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