Exit with status code 2 if no files were given

By convention exit(1) is reserved for general errors, while exit(2)
means command misuse. Stdlib `flags` package invokes `os.Exit(2)`
after printing out the usage message. This commit changes the empty
files list exit code to be aligned with this behavior.

boltbrowser.go

@@ -33,7 +33,7 @@ func main() {
 
 	if flag.NArg() == 0 {
 		flag.Usage()
-		os.Exit(1)
+		os.Exit(2)
 	}
 
 	err = termbox.Init()