Show error details in /servers/NAME/operations/OPERATION_ID

[JP95Git]

GET: /servers/NAME/operations/OPERATION_ID
does not show the cause of an error, it just say "FAILED". That makes troubleshooting very difficult. pg-backup-api should show more details about the error. These details can be found here:
BARMAN_HOME/NAME/output in the file system. There is a json file for each job with all details.

[JP95Git]

BTW: The error was this: #94 (comment)
I fixed the code of pg-backup-api, compiled it again and all my jobs worked fine.