Commands do not propagate errors to the return status

[niooss-ledger]

When some ledgerctl commands fail, an error message can be written but the program returns a successful error code. For example:

$ ledgerctl info
No Ledger device has been found.
$ echo $?
0

$ ledgerctl install-ca ledgerctl
The device is not in recovery mode.
$ echo $?
0

It feels more natural to return an non-zero value for example in

ledgerctl/ledgerctl.py

Line 78 in 3d06e76

sys.exit(0)

Also, the message could be prefixed by something indicating that an error happened (for example Error: No Ledger device has been found. or [ERROR] No Ledger device has been found. or [!] No Ledger device has been found.). What do you think?