[modules/battery] Fix ac and unknown display

If the computer runs on AC, display that instead of showing "100%" in
the status.

Also, if reading the charging status fails for some reason (except the
computer being on AC), go into critical state and display "n/a".

see #23

themes/icons/awesome-fonts.json

@@ -1,5 +1,8 @@
 {
-	"defaults": { "separator": "", "padding": " " },
+	"defaults": {
+		"separator": "", "padding": " ",
+		"unknown": { "prefix": "" }
+	},
 	"date": { "prefix": ""	},
 	"time": { "prefix": "" },
 	"memory": { "prefix": "" },