Fixed errors if acpitool isn't installed on linux

plugins/battery/battery.plugin.zsh

@@ -71,7 +71,9 @@ elif [[ $(uname) == "Linux"  ]] ; then
   }
 
   function battery_pct() {
+    if (( $+commands[acpi] )) ; then
       echo "$(acpi | cut -f2 -d ',' | tr -cd '[:digit:]')"
+    fi
   }
 
   function battery_pct_remaining() {
@@ -147,7 +149,6 @@ function battery_level_gauge() {
     filled_symbol=${BATTERY_UNKNOWN_SYMBOL:-'.'};
   fi
 
-  
   local charging=' ' && battery_is_charging && charging=$charging_symbol;
 
   printf ${charging_color//\%/\%\%}$charging${color_reset//\%/\%\%}${battery_prefix//\%/\%\%}${gauge_color//\%/\%\%}