refactor: improve error handling messages across audio formats

Add explanatory expect() messages when initializing decoders for FLAC, MP3,
and WAV formats. Also improves MP3 decoder's span length handling by removing
unwrap() in favor of the ? operator. These changes maintain the same behavior
while making debugging easier by providing context for potential panics.

Affected decoders:
- FLAC
- MP3
- WAV

Author: roderickvd

src/decoder/flac.rs

@@ -37,7 +37,7 @@ where
             return Err(data);
         }
 
-        let reader = FlacReader::new(data).unwrap();
+        let reader = FlacReader::new(data).expect("should still be flac");
 
         let spec = reader.streaminfo();
         let sample_rate = spec.sample_rate;

src/decoder/mp3.rs

@@ -37,7 +37,7 @@ where
         // thus if we crash here one of these invariants is broken:
         // .expect("should be able to allocate memory, perform IO");
         // let current_span = decoder.decode_frame()
-        let current_span = decoder.next_frame().unwrap();
+        let current_span = decoder.next_frame().expect("should still be mp3");
 
         Ok(Mp3Decoder {
             decoder,
@@ -96,7 +96,8 @@ where
     type Item = DecoderSample;
 
     fn next(&mut self) -> Option<Self::Item> {
-        if self.current_span_offset == self.current_span_len().unwrap() {
+        let current_span_len = self.current_span_len()?;
+        if self.current_span_offset == current_span_len {
             if let Ok(span) = self.decoder.next_frame() {
                 // if let Ok(span) = self.decoder.decode_frame() {
                 self.current_span = span;

src/decoder/wav.rs

@@ -32,7 +32,7 @@ where
             return Err(data);
         }
 
-        let reader = WavReader::new(data).unwrap();
+        let reader = WavReader::new(data).expect("should still be wav");
         let spec = reader.spec();
         let len = reader.len() as u64;
         let reader = SamplesIterator {
@@ -57,6 +57,7 @@ where
             channels: channels as ChannelCount,
         })
     }
+
     pub fn into_inner(self) -> R {
         self.reader.reader.into_inner()
     }