In checkEncryptable(), check whether the directory is already encrypted
before checking whether it's empty.

Also improve the error message for when a directory is nonempty.

Finally, translate keyring.ErrKeyAddedByOtherUsers and
keyring.ErrKeyFilesOpen into errors which include the directory.

Introduce filesystem.ErrEncryptionNotEnabled and
filesystem.ErrEncryptionNotSupported which include the Mount as context,
and translate the corresponding metadata/ errors into them.  Then make
these errors show much better suggestions.

Also replace lots of other filesystem/ errors with either custom types
or with unnamed one-off errors that include more context.  Fix backwards
wrapping in lots of cases.

Finally, don't include the mountpoint in places where it's not useful,
like OS-level errors that already include the path.

ErrBadConfig:
	Fix backwards wrapping, include the bad config, and make it
	clear that this is an internal error.

ErrBadConfigFile:
	Fix backwards wrapping, include the config file location, and
	adjust the suggestion slightly.

ErrConfigFileExists:
	Include the config file location.

ErrNoConfigFile:
	Include the config file location, and adjust the suggestion
	slightly.

Test 'fscrypt setup'.