Starcrawlers: How can I exit a mission partway through?

TLDR: If it's a Story Mission, you can't. You will have to go back to the last save from when you were in town, and start over from there. Save early, and save often.

Missions in Starcrawlers are divided into two types: "Story Missions" and regular missions. In regular missions you can exit any time, at fairly frequent locations. You will see a green 'exit door' line on the map. If you look aound that spot, there should be an object you can interact with called "Exit Hatch". You can use Exit Hatches at any time to leave the mission and return to town. Exiting this way, you will keep all of the XP and loot you have earned so far, but you will not get the bonus reward XP for completing the mission.

When you are in a Story Mission, you can not exit the mission. This is quite unfortunate. Once you start it, the only outcomes are to return to an earlier save; to complete the mission; or to die. If your entire party dies, you will have to start over from a previous save anyway. For this reason, you may want to complete several regular missions before taking on a Story Mission, to level up your party. You may also want to bring along healing supplies such as Medkits.

From the Starcrawler Release Notes, May 2017: "Abort Mission has been removed. To successfully exit a mission, you’ll have to make it to one of several conveniently lit exit doors. No free escapes".