[buildcff2vf] Give better error information when checking compatibility

[punchcutter]

In most normal workflows this shouldn't happen, but I tried to run on source OTFs that had overlaps removed (so totally not compatible) and maybe this error could be better:

File "buildcff2vf.py", line 111, in add_point
    cmd = self._commands[self.pt_index]
IndexError: list index out of range

If the index is out of range then it might be better to catch that and at least report which glyph it is.