Include unknown variant for sample format

Since it is known that libtiff produces other sample format variants we
do not support, represent these as variants. This also allows returning
the sample formats in errors so that composition of formats and known
and unknown formats are clearly reported.

src/decoder/mod.rs

@@ -423,15 +423,13 @@ impl<R: Read + Seek> Decoder<R> {
             self.samples = val;
         }
         if let Some(vals) = self.find_tag_unsigned_vec(Tag::SampleFormat)? {
-            self.sample_format = vals.iter()
-                .map(|&v| SampleFormat::from_u16(v)
-                    .ok_or_else(|| TiffUnsupportedError::UnsupportedSampleFormat(vals.clone()))
-                )
-                .collect::<Result<_, _>>()?;
+            self.sample_format = vals.into_iter()
+                .map(SampleFormat::from_u16_exhaustive)
+                .collect();
 
             // TODO: for now, only homogenous formats across samples are supported.
             if !self.sample_format.windows(2).all(|s| s[0] == s[1]) {
-                return Err(TiffUnsupportedError::UnsupportedSampleFormat(vals).into())
+                return Err(TiffUnsupportedError::UnsupportedSampleFormat(self.sample_format.clone()).into())
             }
         }
         match self.samples {
@@ -905,7 +903,7 @@ impl<R: Read + Seek> Decoder<R> {
                     ))
                 }
             },
-            format => Err(TiffUnsupportedError::UnsupportedSampleFormat(vec![format.to_u16()]).into()),
+            format => Err(TiffUnsupportedError::UnsupportedSampleFormat(vec![format.clone()]).into()),
         }
     }