Better error messages if Kit/track contains nested composite sound

I.e., a composite sound such as [a, [b, c]] is not valid. Previously,
this would result in a cryptic "no implicit conversion of Array to
String" error. Now it results in a more helpful message indicating
which track/Kit sound has the error.

lib/beats/kit_builder.rb

@@ -23,11 +23,14 @@ def add_item(label, filenames)
           raise SoundFileNotFoundError, "Kit has an empty composite pattern (i.e. \"[]\"), which is not valid."
         end
 
-        @composite_replacements[label] = filenames.map {|filename| "#{label}-#{File.basename(filename, ".*")}" }
-
         filenames.each do |filename|
+          unless filename.is_a?(String)
+            raise SoundFileNotFoundError, "The Kit sound '#{label}' contains an invalid file: '#{filename}'"
+          end
           @labels_to_filenames["#{label}-#{File.basename(filename, ".*")}"] = absolute_file_name(filename)
         end
+
+        @composite_replacements[label] = filenames.map {|filename| "#{label}-#{File.basename(filename, ".*")}" }
       else
         @labels_to_filenames[label] = absolute_file_name(filenames)
       end

lib/beats/songparser.rb

@@ -162,6 +162,9 @@ def add_patterns_to_song(song, kit_builder, raw_patterns)
           end
 
           track_names.map! do |track_name|
+            unless track_name.is_a?(String)
+              raise ParseError, "'#{track_name}' in pattern '#{pattern_name}' is not a valid track sound"
+            end
             kit_builder.composite_replacements[track_name] || track_name
           end
           track_names.flatten!

test/fixtures/invalid/kit_with_composite_nested_sounds.txt

@@ -0,0 +1,16 @@
+Song:
+  Tempo: 100
+  Kit:
+    - bass:      test/sounds/bass_mono_8.wav
+    - snare:     test/sounds/snare_mono_8.wav
+    - hh_closed: [test/sounds/hh_closed_mono_8.wav, [test/sounds/hh_open_mono_8.wav]]
+  Flow:
+    - Verse:  x2
+    - Chorus: x2
+
+Verse:
+  - bass:   X...X...
+  - snare:  ..X...X.
+Chorus:
+  - bass:       XXXXXXXX
+  - hh_closed:  .X.X.X.X

test/fixtures/invalid/track_with_composite_nested_sounds.txt

@@ -0,0 +1,16 @@
+Song:
+  Tempo: 100
+  Kit:
+    - bass:      test/sounds/bass_mono_8.wav
+    - snare:     test/sounds/snare_mono_8.wav
+    - hh_closed: test/sounds/hh_closed_mono_8.wav
+  Flow:
+    - Verse:  x2
+    - Chorus: x2
+
+Verse:
+  - [bass, [hh_closed]]:  X...X...
+  - snare:              ..X...X.
+Chorus:
+  - bass:          XXXXXXXX
+  - [snare, [bass, hh_closed]]: .X.X.X.X

test/songparser_test.rb

@@ -16,6 +16,8 @@ class SongParserTest < Minitest::Test
                       :kit_with_composite_non_existent_sound,
                       :track_with_composite_empty_sound,
                       :kit_with_composite_empty_sound,
+                      :track_with_composite_nested_sounds,
+                      :kit_with_composite_nested_sounds,
                       :leading_bar_line,
                       :sound_in_kit_not_found,
                       :sound_in_track_not_found,