Investigate: Error translation if SSH host is down

[buhtz]

A todo note for myself.
Found this behavior in v1.3.3. Need to investigate this in latest dev version.

This happens in case when SSH host is not available (down). This should not be an error but just a message, when starting BIT GUI manually. Because this is not an usual case.

This is the related code.

backintime/common/sshtools.py

Line 519 in b9dbc30

raise NoPubKeyLogin(

Additionally it seems I removed its translation.