If a chordsym has an alter for a degree not in the chord, turn it int…

…o an add. I'm assuming this is what "should we throw an exception???? for now yes, but maybe later we should" is referring to.

music21/harmony.py

@@ -1824,7 +1824,12 @@ def typeAlter(hD):
             elif chordStepModification.modType == 'subtract':
                 typeSubtract(chordStepModification)
             elif chordStepModification.modType == 'alter':
-                typeAlter(chordStepModification)
+                try:
+                    typeAlter(chordStepModification)
+                except ChordStepModificationException:
+                    # fix it in place
+                    chordStepModification.modType = 'add'
+                    typeAdd(chordStepModification)
 
         return pitches