Error messages for Collada Validate() functions. 

This is a first proposal to improve the validation step in OpenCollada. Currently avalidation failure results in silent drop of the invalid data. With this patch the error is reported. I have also added a verbose flag for future use (currently set to true).

If this is a convenient proposal, then i can add 2 more features :

1.) Add a flag that controls "loose" vs."strict" validation. "

    - "loose" would mean: the data is not consistent, but it probably could be imported.
    - "strict" would mean: Report any inconsistency as "Error" and discard the data.

2.) Add the ability to control the validation type (strict/loose) and the verbosity from the API.

Here i would need some guideline how this should be implemented.