Prevalidate Nif data before export

[Dexesttp]

Non user-friendly messages on export fail.

Description of the problem

When exporting a file to a given Nif format, errors can be thrown if data is missing for the Nif building (as described while reviewing PR #225 ).

The errors aren't user friendly, and in addition only the first error is thrown. Thus, the way to correct these errors is usually long and tedious.

Proposed solution

The way to solve this would be to create a method to handle checking the scene data and maybe link this behavior to a "validate" button instead of the export button.
This way, the file validity can be checked before having errors at export. With a custom method, the plugin could also display a number of possible fixes for each problem encountered.

Throwing an exception on export time is not a nice way to inform the user that the blender data can not correctly be mapped to nif data. The check on the scene should in the best case not stop on the first error but write all errors to blender text data, or to a nicely formatted error on import time.

So, to compare the "current" workflow to the proposed :

• edit data in blender -> try to export file -> error -> figure out what is wrong out of the exception -> goto 1

Proposed:

• edit data in blender -> hit validate button -> look at the text -> fix all listed errors -> export nif

Converting between different nif versions becomes easier too

• import morrowind nif -> set scene version to skyrim -> hit validate button -> fix all errors -> export nif

@Eli2 , about this issue