While koji tries to verify the data in CG imports, it is still possible to provide bad data that will result in the import failing after writing files to disc. These stray files are untracked, and prevent further imports of the same nvr.

For now, we should take the following measures:
1) The cg import code should fail early if the build directory already exists
2) The cg import code should have an exception handler that removes the build dir if an error occurs after we start writing