More helpful error message if kit sound's filename is not a String

Previously this would result in an unhelpful "no implicit conversion
of Integer into String" error message. The other scenarios in which
a non-String filename is possible (in a kit composite sound or a
filename directly referenced in a track name) already showed a more
helpful error message.

Author: jstrait

lib/beats/kit_builder.rb

@@ -34,6 +34,10 @@ def add_item(label, filenames)
           @composite_replacements[label] << composite_replacement
         end
       else
+        unless filenames.is_a?(String)
+          raise SoundFileNotFoundError, "Kit sound '#{label}' has an invalid filename: '#{filenames}'. It must be a value that will be parsed from YAML as a String."
+        end
+
         @labels_to_filenames[label] = absolute_file_name(filenames)
         @composite_replacements.delete(label)
       end

test/fixtures/invalid/kit_filename_not_a_string.txt

@@ -0,0 +1,17 @@
+# Invalid song, the 'snare' Kit sound has an invalid filename
+Song:
+  Tempo: 120
+  Flow:
+    - Verse:   x2
+  Kit:
+    - bass:       test/sounds/bass_mono_8.wav
+    - snare:      3
+    - hh_closed:  test/sounds/hh_closed_mono_8.wav
+    - agogo:      test/sounds/agogo_high_mono_8.wav
+
+Verse:
+  - bass:       X...X...X...X...
+  - snare:      ..............X.
+  - hh_closed:  X.XXX.XX........
+  - hh_closed:  ........X.X.X.X.
+  - agogo:      ..............XX

test/song_parser_test.rb

@@ -32,6 +32,7 @@ class SongParserTest < Minitest::Test
                       :track_with_composite_nested_sounds,
                       :kit_with_composite_nested_sounds,
                       :kit_not_an_array,
+                      :kit_filename_not_a_string,
                       :leading_bar_line,
                       :sound_in_kit_not_found,
                       :sound_in_track_not_found,