During migration, fail gracefully with unreleased repos

[cottsay]

This will prevent a KeyError from causing the entire script to exit when encountering a repository release mapping which has no version, such as this one:

rosdistro/rolling/distribution.yaml

Lines 1150 to 1166 in 757cd19

	release:
	packages:
	- swri_console_util
	- swri_dbw_interface
	- swri_geometry_util
	- swri_image_util
	- swri_math_util
	- swri_opencv_util
	- swri_prefix_tools
	- swri_roscpp
	- swri_route_util
	- swri_serial_util
	- swri_system_util
	- swri_transform_util
	tags:
	release: release/rolling/{package}/{version}
	url: https://github.com/swri-robotics-gbp/marti_common-release.git