Resolve "possibly unbound" error

[EarlMilktea]

PR for grover.py

Problem description

Variable n can be unbound if len(sys.argv) is neither 1 nor 3.

import sys

if len(sys.argv) == 1:
    n = 4
    target = q.util.str2binary("0101")
elif len(sys.argv) == 3:
    n = int(sys.argv[1])
    target = q.util.str2binary(sys.argv[2])
# NG!

Fix

raise ValueError if that occurs.