Removed duplicate code

Tests/test_file_palm.py

@@ -46,6 +46,13 @@ class TestFilePalm(PillowTestCase):
         self.skipKnownBadTest("Palm P image is wrong")
         self.roundtrip(mode)
 
+    def test_l_ioerror(self):
+        # Arrange
+        mode = "L"
+
+        # Act / Assert
+        self.assertRaises(IOError, self.helper_save_as_palm, mode)
+
     def test_rgb_ioerror(self):
         # Arrange
         mode = "RGB"

src/PIL/PalmImagePlugin.py

@@ -130,26 +130,23 @@ def _save(im, fp, filename):
         bpp = 8
         version = 1
 
-    elif im.mode == "L" and im.encoderinfo.get("bpp") in (1, 2, 4):
-
+    elif im.mode == "L":
+        if im.encoderinfo.get("bpp") in (1, 2, 4):
             # this is 8-bit grayscale, so we shift it to get the high-order bits,
             # and invert it because
             # Palm does greyscale from white (0) to black (1)
             bpp = im.encoderinfo["bpp"]
             im = im.point(
                 lambda x, shift=8-bpp, maxval=(1 << bpp)-1: maxval - (x >> shift))
-        # we ignore the palette here
-        im.mode = "P"
-        rawmode = "P;" + str(bpp)
-        version = 1
-
-    elif im.mode == "L" and im.info.get("bpp") in (1, 2, 4):
-
+        elif im.info.get("bpp") in (1, 2, 4):
             # here we assume that even though the inherent mode is 8-bit grayscale,
             # only the lower bpp bits are significant.
             # We invert them to match the Palm.
             bpp = im.info["bpp"]
             im = im.point(lambda x, maxval=(1 << bpp)-1: maxval - (x & maxval))
+        else:
+            raise IOError("cannot write mode %s as Palm" % im.mode)
+
         # we ignore the palette here
         im.mode = "P"
         rawmode = "P;" + str(bpp)