Some file operations return undocumented UVError

[eschnett]

I assumed that failing file operations (rm, 'mkdir, etc.) return a SystemError when they fail. However, under certain circumstances they return an undocumented UVError. This seems like an unintentional leak of an implementation detail:

julia> try rm("a") catch err info(typeof(err)) end
INFO: Base.UVError
julia> try chmod("a", 0) catch err info(typeof(err)) end
INFO: Base.UVError

The printed error messages look fine, but e.g. the error codes have different meanings (SystemError uses errno, UVError uses its own set of constants). Having different exception types makes it difficult to handle these errors programmatically.

[IainNZ]

Duplicate of #7841, I think

[samoconnor]

See also #14972