Overiding error message when using a python program

[aleksander.helgaker at gmail.com]

I've completely rewritten a calculator I wrote to help me learn Python.
After someone told me about the def command I reliesed that I could
make the program much better, but there is a very anoying problem which
ocours when I run the program.

Here is the code
<code>
# IMPOSRTS #
import sys
import os

# DEF'S #
def print_intro():
	os.system('clear')
	print "Welcome to Calc v 0.1a"
	print "----------------------"

def main():
	print_intro()
	while True:
		prompt_user()

def prompt_user():
	userinput = input(">")

def fib(n): # write Fibonacci series up to n
	"""Print a Fibonacci series up to n."""
	a, b = 0, 1
	while b < n:
		print b,
		a, b = b, a+b
		print

def quit():
	sys.exit()

# PROGRAM FLOW
main()
</code>

Now when I run this program and I type in a command which I have no
code for e.g. "pi" (which is 3,14....) I get the error message
"NameError: name 'pi' is not defined" and then the program quits.

I'm creating this program for my own use but naturally sometimes I
would make spelling mistakes (being a dyslexic and all) and so having a
long error message and having the program quit is more then a bit
irritating. It would be much more preferable if the program simply
wrote "Command not recognised" and then kept going. Is this possible?