Fixes an incorrect check of backupPath failure return code

backupPath() return value were tested to determine if something went wrong but
only one of the tree error status values were tested (id. 0): Both -1 and 1 were
ignored leaving error cases undetected.

Now, any value other than 2 is considered as "something went wrong".

Closes: #8
Merge branch 'bugfix/8-could_not_backup_still_no_errors'