id3-album.rb: fail early if no .mp3 files found

id3-album.rb

@@ -3,6 +3,12 @@
 # Install: kid3-common
 
 $stdout.sync = true
+files = Dir.glob("*.mp3").sort
+
+if files.empty?
+  $stderr.puts "No .mp3 files found"
+  exit 1
+end
 
 $stdout.write("Artist: ")
 artist = gets.chomp
@@ -13,7 +19,6 @@ album = gets.chomp
 $stdout.write("Year: ")
 year = gets.chomp
 
-files = Dir.glob("*.mp3").sort
 files.each do |file|
   if file =~ /^(\d+)\s*(.*)\.mp3$/
     track_number, title = $1.to_i, $2