Implement core::error::Error for all error types

[Robbepop]

This way we no longer have to conditionally compile for std.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 71.41%. Comparing base (9403447) to head (6ff423f).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1462      +/-   ##
==========================================
+ Coverage   71.40%   71.41%   +0.01%     
==========================================
  Files         162      162              
  Lines       16420    16420              
==========================================
+ Hits        11724    11727       +3     
+ Misses       4696     4693       -3     

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