Post root_validator only called when all field validations succeeded

[PPakalns]

Feature Request

Please complete:

• OS: Linux, Ubuntu
• Python version import sys; print(sys.version): 3.7
• Pydantic version import pydantic; print(pydantic.VERSION): 1.1.1

If field validation fails there often is no need to call root validator as it will fail too because fields are not available.

I suggest that new parameter is added to root_validator decorator that disables call of root_validator if field validation failed.

@root_validator(pre=False, call_on_field_failure=False)

See relevant quote from documentation about current behavior:

Field validation will not occur if pre=True root validators raise an error. As with field validators, "post" (i.e. pre=False) root validators will be called even if field validation fails; the values argument will be a dict containing the values which passed field validation and field defaults where applicable.
From https://pydantic-docs.helpmanual.io/usage/validators/

Thank you all for developing this library!

[samuelcolvin]

Shouldn't be too hard to implement, PR welcome.

Would involve a change here:

https://github.com/samuelcolvin/pydantic/blob/151143deb4eaf6a5e2dce4532659e5ee9e2e4713/pydantic/main.py#L855-L860

call_after_failures argument would need to have default True at least until v2.

[aviramha]

Implemented on #1050. I thought about using the Validator class, but that'd require many changes (right now in the flow @samuelcolvin mentioned the validator config are removed and only functions are left).
The alternative was to add a variable to the function itself, which is what I chose to do.
Another approach which we'll require more coding will change model.post_root_validators and the pre one contain the f_cls instead of only functions.
@samuelcolvin What do you think? Should I go for the other approach or this one suffices?