added new features

- expanding `environment` vars in cmd section
- support for connecting to one proxy/bastion host
- better notification text layout
- better error message on private key failing to open

pkg/backy/templates/error.txt

@@ -0,0 +1,8 @@
+Command list {{.listName }} failed on running {{.Command}}.
+
+The error was {{ .Err }}
+
+The following commands ran:
+{{- range .CmdsRan}}
+    - {{. -}}
+{{end}}