add monitoring script to check errors in logs

When a backup i made without error the output logged is like the following :

using parent snapshot 096ed3f4
scan [/]
[0:20] 32117 directories, 186610 files, 5.074 GiB
scanned 32117 directories, 186610 files in 0:20
[1:00] 100.00%  0B/s  5.074 GiB / 5.074 GiB  218726 / 218727 items  0 errors  ETA 0:00 

duration: 1:00, 85.62MiB/s
snapshot 5125bb88 saved

The idea is to simple grep the pattern '\s0\serrors\s' and being compliant with nagios like tools :

• if the log file exists and pattern is present in the file, returns 0
• if the log file exists and pattern is not present in the file, return 2
• if the log file does not exists, is not readable, does not contains word snapshot , return 3