The word "forgiving" is one of the most used for this context. It's slightly informal, but could easily be used in more formal situations if, for example, you were listing qualities about the programming language or writing a short paragraph about it. "Forgiving" (in the context of programming or user input) means something is more accepting of small errors made by the user, and more friendly to users with less experience.
"Robust" is a more formal word, and can be issued as a general statement about something, but is typically reserved for cases where something has multiple qualities that can be described this way, and that can stand up to the rigors of testing.
"It's a very forgiving programming language."
"The software is very forgiving to coding errors."
"The software is very forgiving to the user."