The `error_coercer` filled in at engine initialization isn't taken into account

[Maximilien Raulic]

While error_coercer parameter from the create_engine utility function or cook method from Engine class is properly taken into account, the error_coercer from the __init__ method of the Engine class isn't.

• Tartiflette version: 1.0.0
• Python version: 3.7
• Executed in docker: No