PIM-3369 : stops the product import with a detailed notice when
encounters issues with field formatting

Conflicts:
	CHANGELOG.md
PIM-3369 : fix issue with optional currency in a csv field
PIM-3369 : fix the optional two fields case for metric import
PIM-3369 : add scenario on invalid headers
Merge pull request #1831 from akeneo/PIM-3369

PIM-3369 : stops the product import with a detailed notice when issue with fields format