Improve error message when fields don't match dataset.

Currently if you upload a file that doesn't include a required column, you
just get an error like "KeyError: 'id'". This patch improves the error message
so that the problem is easier to understand and solve.
Improve error message when fields don't match dataset.

Currently if you upload a file that doesn't include a required column, you
just get an error like "KeyError: 'id'". This patch improves the error message
so that the problem is easier to understand and solve.
Commit a blind fix for Python 3.3 errors.

https://travis-ci.org/bmihelac/django-import-export/jobs/28887412
Merge branch 'fix_missing_field_error_message'

Conflicts:
	import_export/fields.py
Merge branch 'master' of https://github.com/bmihelac/django-import-export
Merge branch 'allow_override_resource_field_construction'
Another blind fix for python 2/3 issues.
Merge branch 'fix_missing_field_error_message'
Merge branch 'master' of https://github.com/bmihelac/django-import-export
Merge branch 'fix_link_to_nonrelation_field_error_message_2'
Improve error message when Resource names a nonexistent field.

When someone creates a Resource referencing a field that doesn't exist
via a ForeignKey:

	class BrokenBook(resources.ModelResource):
	    class Meta:
	        model = Book
	        fields = ('author__nonexistent',)

We replace the standard Django error message:

	FieldDoesNotExist: Author has no field named u'nonexistent'

With a message that shows the path to the field that didn't exist (and leaves
out the spurious "u"):

	FieldDoesNotExist: Book.author.nonexistent: Author has no field named 'nonexistent'
Merge branch 'fix_link_to_nonexistent_field_error_message'

Conflicts:
	import_export/resources.py
Fix Python 3 by not mixing unicode and byte strings.
Merge commit '452ff454a0f9aad06a018f31dccc75ccd12e67c6'
Merge pull request #112 from aptivate/fix_link_to_nonexistent_field_error_message

Improve error message when Resource names a nonexistent field.
Merge pull request #109 from aptivate/fix_missing_field_error_message

Improve error message when fields don't match dataset.
Merge branch 'master' of https://github.com/bmihelac/django-import-export
Add column name to ValueError from Field.clean()

It's useful to know which column failed to validate, and not always obvious.
This change adds the column name to the ValueError message, for example:

	Column 'id': invalid literal for int() with base 10: 'foo'
Merge branch 'add_column_name_to_clean_error_message'
Merge pull request #113 from aptivate/add_column_name_to_clean_error_message

Add column name to ValueError from Field.clean()