Truncate fail_reason to column length

During create_backup failure handling, backup_update fails with
DataError ("Data too long for column") if the fail_reason is
greater than 255 characters. As a result, backup status is stuck
in 'creating' state. This patch avoids the problem by truncating
fail_reason to 255 characters before update.

Closes-Bug: #1376199
Change-Id: If0d616b81d3869f7ea110caab8cf4140cf5c5c9e
(cherry picked from commit 82716b4ac836024b968c76db75be8a92ede0e226)
Merge "Truncate fail_reason to column length" into stable/juno