raise exception instead of sys.exit

[bertsky]

Printing and forced system exit makes it difficult to wrap this as a library, and even on the interactive interpreter it is quite a surprise to find yourself kicked out:

git-lfs-fetch.py/git_lfs/__init__.py

Lines 159 to 162 in 23026ff

	if checkout_dir == git_dir:
	print('Can\'t checkout into a bare repo, please provide a valid '
	'checkout_dir')
	raise SystemExit(1)

So IMO this should simply be an assertion or exception.

[Changaco]

I encourage you to make a pull request to fix this issue.