preserve internal error

[tpolecat]

We ran into an issue where an internal error in SqlCursor.field "expected exactly one row (or something like that)" was masked by orElse selecting the generic mkCursorForField, which then failed with "Unhandled mapping of type grackle.sql.SqlMappingLike$SqlField", which is misleading. This PR fixes the specific issue, but I think in general we may want a combinator like r1 onFailure r2 that only selects r2 in the case of logical failure, preserving r1 as-is in case of InternalError.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 75.62%. Comparing base (3a6b7b8) to head (adc3c07).
Report is 75 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #646      +/-   ##
==========================================
+ Coverage   74.22%   75.62%   +1.39%     
==========================================
  Files          32       32              
  Lines        4609     5033     +424     
  Branches     1025     1083      +58     
==========================================
+ Hits         3421     3806     +385     
- Misses       1188     1227      +39     

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[milessabin]

LGTM!