Excluding a file in stage but including it in prime raises an exception

[Leo Arias]

When a snapcraft.yaml excludes a file with the stage keyword, but includes it with the prime keyword, a FileNotFoundError exception is raised.

To reproduce:
mkdir test
cd test
touch conflict-file
Download the attached snapcraft.yaml file and save it in the test directory
Run snapcraft
[...]
FileNotFoundError: [Errno 2] No such file or directory: '/home/ubuntu/tmp/test/stage/conflict-file' -> '/home/ubuntu/tmp/test/prime/conflict-file'
[...]

Full log:
http://paste.ubuntu.com/26031810/

[Leo Arias (elopio)]

There is an exception in snapcraft/internal/errors.py called PrimeFileConflictError, that apparently should be raised in this case.