Clarify the "Non accessible element" error

This error happens, among others, when trying to set an element as if it was a
parameter. The error message was not very helpful.

Previous error message:
        Non accessible element

New error message:
       Can't set /A/b/c because it is not a parameter

Signed-off-by: David Wagner <david.wagner@intel.com>
Merge pull request #23 from 01org/clarify-non-accesibble-error

Clarify the "Non accessible element" error