Handle boundary edge components and refactor iter_dem_instructions_include_correlations

[oscarhiggott]

Fixes a bug where boundary edges weren't always handled properly when they were a component of an error. i.e. handle_dem_error was called the same on edges and boundary edges:

} else if (num_component_detectors > 0) {
    // If the previous error in the decomposition had 1 or 2 detectors, we handle it
    handle_dem_error(p, {component->node1, component->node2}, component->observable_indices);

This resulted in an exception if the boundary edge was not already in the graph. Also refactors related logic slightly.

[smadhuk]

LGTM