I installed "Minions 1.7.2" (properly; I know what I'm doing) and decided I did not like it. I deleted the 2 mod files that were required to be placed in the /mods/ folder, and the 2 files in the /config/ folder (1 placed, 1 created by playing the game). And now I get the same type of error message.
I unsummoned the minions. I threw the "Master's" (whatever) into lava, so it's gone. I picked up the chest the Minions were supposed to put their mined materials into. I regenerated (with mcedit) the (2) chunks that the minions failed to put a mineshaft between. There's now no visible sign in-game that they were ever there.
How do I fix this error message?
Solution: Edit the profile to use a default version of Minecraft. (I used 1.7.2 and it fixed my world save. The world save loaded just fine in Minecraft 1.7.2, and when I switched back to my mod'd profile, everything was back to normal.)