This is an implementation of BankAccounts wave one. Our task today will be to change the behavior of Bank::Account#withdraw.

As things stand, when a withdrawal would make the account balance go negative, the program puts a message to the screen and returns the un-modified balance. Instead, we should raise an exception.

Work will proceed in 3 stages:

1. Change the tests for Bank::Account#withdraw to expect an exception rather than a puts
2. Define a new exception, InsufficientFundsError, in a new file lib/insufficient_funds_error.rb. InsufficientFundsError should inherit from StandardError.
3. Modify the #withdraw method to raise the exception rather than calling puts