Raise exceptions in exporters instead of returning errors

[olivier]

In many places, you return or yield error strings from the exporters methods. These errors can't be properly caught by the Python error handling mechanism, and it result in silent failures. In Python (and other OO language), you don't return errors, you must raise exceptions.

More about exceptions:
​http://www.python.org/doc/current/tut/node10.html

[anonymous]

In r126, I see you call raise with strings... This isn't right. String exceptions are a bad practice, they can't be properly caught and are deprecated in Python 2.5.

Please use class exceptions, as explained in "8.4 Raising Exceptions" and "8.5 User-defined Exceptions", in the tutorial page I mention in my report.

[olivier]

By the way, if you need to define custom ("User-defined") exceptions, you should put the class definitions in export/api.py, so that one knows what exceptions to expect from the export package, what specific attributes these exceptions carry, etc..