Optional error message in ValidateXXX functions

[Yuri05]

At the moment, validate functions do not provide any information about their execution context.
If validation fails, the user just gets an error message "variable XY has wrong length/type/...", e.g.

> validateIsOfLength(appliedCompoundName, 1)
Error in validateIsOfLength(appliedCompoundName, 1) : 
  validateIsOfLength: Object should be of length '1', but is of length '2' instead. 

In order to improve usability, I would extend all these function with additional (optional) string argument - error message, which would be shown to the user instead of (or additionally to?) the current error message.