cbrestore - Display informative error message when restoring from cbbackupmgr backup

Currently, when a user attempts to restore a cbbackupmgr backup using cbrestore, it displays a generic error.

For example:

"error: no valid backup directory found: 2018-09-07T11_26_33.989534+01_00/testbucket-770011382af1dcffd466fec87eba3302"

or

"error: not a bucket directory: testbucket-770011382af1dcffd466fec87eba3302/bucket-config.json".  

It would be useful to users if cbrestore was able to detect a backup directory as one generated by cbbackupmgr and advise the user with a "error: backup directory was generated with cbbackupmgr" or similar.