Update adafruit_midi/midi_message.py to match suggestion

Co-authored-by: Kattni <kattni@adafruit.com>

Author: evaherrada

adafruit_midi/midi_message.py

@@ -111,7 +111,7 @@ def channel(self):
     @channel.setter
     def channel(self, channel):
         if channel is not None and not 0 <= channel <= 15:
-            raise "Channel must be 0-15 or None"  # pylint: disable=raising-bad-type
+            raise ValueError("Channel must be 0-15 or None")
         self._channel = channel
 
     @classmethod