fix: Export RemoveDeadFuncsError

[aborgna-q]

The RemoveDeadFuncsPass returns this error, which was hidden inside a private module.

Adds missing docs to the struct, and converts the tuple variant into a regular struct.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 86.55%. Comparing base (e26a31f) to head (09bd748).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
hugr-passes/src/dead_funcs.rs	0.00%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1883   +/-   ##
=======================================
  Coverage   86.55%   86.55%           
=======================================
  Files         195      195           
  Lines       35584    35584           
  Branches    32397    32397           
=======================================
  Hits        30800    30800           
  Misses       3012     3012           
  Partials     1772     1772           

Flag	Coverage Δ
python	92.34% <ø> (ø)
rust	85.98% <0.00%> (ø)

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[doug-q]

Thanks!