Fix more than one band error (#234)

Precompute band count before closing the file
Fixes #233

stackstac/rio_reader.py

@@ -335,9 +335,10 @@ def _open(self) -> ThreadsafeRioDataset:
 
                     raise RuntimeError(msg) from e
             if ds.count != 1:
+                nr_of_bands = ds.count
                 ds.close()
                 raise RuntimeError(
-                    f"Assets must have exactly 1 band, but file {self.url!r} has {ds.count}. "
+                    f"Assets must have exactly 1 band, but file {self.url!r} has {nr_of_bands}. "
                     "We can't currently handle multi-band rasters (each band has to be "
                     "a separate STAC asset), so you'll need to exclude this asset from your analysis."
                 )