Python 3.x 如果用户输入的值与程序所需的值（a、b等）不同，我想排除错误

我尝试了put Exception Keyerror:或Exception nothing nothing True work python continue如果我输入了一个回溯（最近一次调用last），则给出此错误： KeyError:'a'

roman_to_decimal = { 'I': 1,'i':1,'v':5 , 'V': 5,'x':10, 'X': 10,'l':50, 'L': 50,'c':100, 'C': 100, \
                         'd':500,'D': 500, 'm':1000,'M': 1000 }
    #decimal to roman 
    def int2roman(numberdec):
        numerals={1:"I", 4:"IV", 5:"V", 9: "IX", 10:"X", 40:"XL", 50:"L",
                  90:"XC", 100:"C", 400:"CD", 500:"D", 900:"CM", 1000:"M"}
        result=""
        for value, numeral in sorted(numerals.items(), reverse=True):
            while numberdec >= value:
                result += numeral
                numberdec -= value
        return result
    while True:
        try:  
            numberchk=(input("Enter a Roman numeral or a Decimal numeral:" ))
            break
#the problem is here 
    except:
        print ("Oops!  That was no valid numeral.  Try again...")

while True:
    try:  
        numberchk = input("Enter a Roman numeral or a Decimal numeral: ")
        if all(letter in roman_to_decimal for letter in numberchk.strip()):
            print('doing something with roman')
            break
        elif numberchk.strip().isdigit():
            print('doing something with decimal')
            break
        else:
            errmsg = "'{}' is not decimal or roman numeral.".format(numberchk)
            raise ValueError(errmsg) # errmsg for debugging
    except (ValueError):
        print ("Positive decimal or roman numeral was expected. Try again...")