Checking parameters

[Cecil Westerhof <dummy@dummy.nl>]

I am new to Emacs and lisp.
What is the best way to check parameters and do error handling when they are
not correct?

I have the following function:
(defun getHours(time)
  (interactive "sHH:MM: ")
  (let ((timelist (split-string time ":")))
    (+
      (string-to-number (car timelist))
      (/
        (string-to-number (cadr timelist))
        60.0
      )
    )
  )
)

How do I check that there is exactly one parameter? And how do I check the
format and give an error that works in interactive and normal mode?
For example when I give
    (getHours "0:120")
I get
    2.0
How should I generate an error/exception?