An error returned by a Linx.Capabilities read operation.

Built from a failed File.read/1 against /proc/<pid>/status, or from a parser that couldn't make sense of the contents. The shape:

• :path — the procfs path the operation targeted.
• :operation — what we were trying to do, as an atom. The read path uses :read; future operations would extend this.
• :errno — the POSIX errno as an atom (:enoent, :eacces, …), or :bad_status if the file existed but didn't contain the five Cap*: lines we expected.
• :code — the matching positive errno integer, or nil for atoms outside the POSIX table (:bad_status has code: nil).

Pattern-match on :errno and :operation to handle specific failures:

case Linx.Capabilities.read(pid) do
  {:ok, state} ->
    inspect(state)

  {:error, %Linx.Capabilities.Error{errno: :enoent}} ->
    # Target pid is gone.
    :pid_dead

  {:error, %Linx.Capabilities.Error{errno: :eacces}} ->
    # No permission to read /proc/<pid>/status — should be
    # rare; status is usually world-readable.
    :no_perm

  {:error, %Linx.Capabilities.Error{errno: :bad_status}} ->
    # The file existed but didn't have the expected Cap*:
    # lines. Should never happen on a real Linux kernel.
    :unparseable
end

Pre-exec failures from the write side don't surface as %Linx.Capabilities.Error{} — they come through Linx.Process's {:linx_process, :error, errno, stage} shape with stage atoms like :cap_drop_bounding. This struct is for the host-side read path only.

Implements Exception, so an error can be raised or rendered with Exception.message/1.