Use display rather than description for DmError::Core errors
Use display rather than description for DmError::Core errors

Due to error_chain rules, the display is fuller and more interesting than
the description.

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

stratis-storage/stratisd,trgill/stratisd,mulkieran/stratisd,stratis-storage/stratisd,mulkieran/stratisd,stratis-storage/stratisd,trgill/stratisd