Small improvements to error message wording

In particular, the new wording includes the common name of the
relevant format (i.e. WAVE_FORMAT_EXTENSIBLE), since the format
code of 65534 might be cryptic on its own.

lib/wavefile/chunk_readers/format_chunk_reader.rb

@@ -35,7 +35,7 @@ def read
 
           if format_chunk[:audio_format] == FORMAT_CODES[:extensible]
             if format_chunk[:extension_size] < 22
-              raise_error InvalidFormatError, "The format chunk extension size of #{format_chunk[:extension_size]} bytes is too small. Since this format chunk has a format code of #{FORMAT_CODES[:extensible]}, the extension must be at least 22 bytes long."
+              raise_error InvalidFormatError, "The format chunk extension size of #{format_chunk[:extension_size]} bytes is too small. Since the format chunk has a format code of #{FORMAT_CODES[:extensible]} (i.e. WAVE_FORMAT_EXTENSIBLE), the extension size must be at least 22 bytes."
             end
 
             format_chunk[:valid_bits_per_sample] = raw_bytes.slice!(0...2).unpack(UNSIGNED_INT_16).first