dials.scale exits on error without traceback

[dagewa]

This is spun out from xia2/xia2#430

export_mtz from command_line/export_mtz.py is treated as library code by dials.scale:

dials/command_line/scale.py

Line 148 in 5058b42

export_mtz(export_params, experiments, [reflection_table])

However a ValueError raised here is caught and the program simply exits, which is confusing

dials/command_line/export.py

Lines 263 to 266 in 5058b42

	try:
	m = export_mtz(reflections[0], experiments, params)
	except ValueError as e:
	sys.exit(e)

Maybe export_mtz should not be swallowing this ValueError, or/and maybedials.scale should be using dials.util.export_mtz instead.