Convert a numbered vowel to an accented vowel.

def _num_vowel_to_acc(vowel, tone):
    """Convert a numbered vowel to an accented vowel."""
    try:
        return VOWEL_MAP[vowel + str(tone)]
    except IndexError:
        raise ValueError("Vowel must be one of '{}' and tone must be a tone.".format(VOWELS))