Display different backup failure message when the COPY strategy is used

Summary

When using sudo gitlab-rake gitlab:backup:create STRATEGY=copy we don't know if it failed due to the rsync or tar commands as the Error message Backup failed is the same for both.

See lib/backup/files.rb

Improvements

Alter the error message to distinguish which operation failed, this would help speed up the analysis of the failure.

Risks

None

Involved components

lib/backup/files.rb