Translate encoder error codes to strings

When decoding, we use raise_oserror() to convert codec error codes to
strings.  Adapt that code to be used when encoding as well.  Add a new
helper function that returns the exception so we can still raise
`from exc`.

docs/releasenotes/10.2.0.rst

@@ -77,3 +77,9 @@ Calculating the :py:attr:`~PIL.ImageStat.Stat.count` and
 :py:attr:`~PIL.ImageStat.Stat.extrema` statistics is now faster. After the
 histogram is created in ``st = ImageStat.Stat(im)``, ``st.count`` is 3x as fast
 on average and ``st.extrema`` is 12x as fast on average.
+
+Encoder errors now report error detail as string
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+:py:exc:`OSError` exceptions from image encoders now include a textual description of
+the error instead of a numeric error code.

src/PIL/ImageFile.py

@@ -63,15 +63,19 @@
 # Helpers
 
 
-def raise_oserror(error):
+def _get_oserror(error, *, encoder):
     try:
         msg = Image.core.getcodecstatus(error)
     except AttributeError:
         msg = ERRORS.get(error)
     if not msg:
-        msg = f"decoder error {error}"
-    msg += " when reading image file"
-    raise OSError(msg)
+        msg = f"{'encoder' if encoder else 'decoder'} error {error}"
+    msg += f" when {'writing' if encoder else 'reading'} image file"
+    return OSError(msg)
+
+
+def raise_oserror(error):
+    raise _get_oserror(error, encoder=False)
 
 
 def _tilesort(t):
@@ -551,8 +555,7 @@ def _encode_tile(im, fp, tile: list[_Tile], bufsize, fh, exc=None):
                     # slight speedup: compress to real file object
                     errcode = encoder.encode_to_file(fh, bufsize)
             if errcode < 0:
-                msg = f"encoder error {errcode} when writing image file"
-                raise OSError(msg) from exc
+                raise _get_oserror(errcode, encoder=True) from exc
         finally:
             encoder.cleanup()