Swap $input and $exists in init_cache_generate_error_string
Swap $input and $exists in init_cache_generate_error_string

This fixes a bug in the error string generated when an attempt is made
to initialize a cache when the cache already exists but the devices
specified are not the same as those in the existing cache.

Signed-off-by: mulhern <7b51bcf507bcd7afb72bf8663752c0ddbeb517f6@redhat.com>

stratis-storage/stratisd,stratis-storage/stratisd,stratis-storage/stratisd