Support Commanded's event handler error/3 callback

[slashdotdash]

Allow projectors to return an error tagged tuple ({:error, error}) from a project macro which will cause an error/3 callback function to be called, if defined.

Exceptions when executing the Ecto.Multi will also be rescued and converted into {:error, exception} tuples allowing you to handle in an error/3 function.

Example usage

defmodule ErrorProjector do
  use Commanded.Projections.Ecto, name: "ErrorProjector"

  alias Commanded.Event.FailureContext

  project %ErrorEvent{name: name} do
    {:error, :failed}
  end

  # Skip failed event on error
  def error({:error, _error}, %ErrorEvent{}, %FailureContext{}) do
    :skip
  end
end

Fixes #10.