Tidy up BatteryLevel() function.

1. Remove assert() around io.popen() as this function never fails, even
when opening a pipe to a non-existent program.
2. Remove the second assert() as well, because it is not wise to fail the whole
application just because we don't know or cannot ascertain the battery
level.
3. Guard against the failure of gasgauge-info (e.g. on emulator where it
does not exist, so, effectively "fails") by checking what we read from
the pipe and return "?" if need be.

filechooser.lua

@@ -56,9 +56,9 @@ end
 
 function BatteryLevel()
 	-- NuPogodi, 18.05.12: This command seems to work even without Amazon Kindle framework 
-	local cmd="gasgauge-info -s 2> /dev/null"
-	local p = assert(io.popen(cmd, "r"))
-	local battery = assert(p:read("*a"))
+	local p = io.popen("gasgauge-info -s 2> /dev/null", "r") -- io.popen() _never_ fails!
+	local battery = p:read("*a") or "?"
+	if battery == "" then battery = "?" end
 	p:close()
 	return string.gsub(battery, "[\n\r]+", "")
 end