Validation of fields in tagged unions creates confusing errors

[ojii]

Checks

• I added a descriptive title to this issue
• I have searched (google, github) for similar issues and couldn't find anything
• I have read and followed the docs and still think this is a bug

Bug

Output of python -c "import pydantic.utils; print(pydantic.utils.version_info())":

             pydantic version: 1.8.2
            pydantic compiled: False
                 install path: /Users/jonas.obrist/playground/pydantic/pydantic
               python version: 3.8.3 (default, Jul 17 2020, 11:57:37)  [Clang 10.0.0 ]
                     platform: macOS-10.16-x86_64-i386-64bit
     optional deps. installed: ['devtools', 'dotenv', 'email-validator', 'typing-extensions']

Tested with git checkout 5ccbdcb.

See this test in my branch.

from typing import Literal, Union

from pydantic import BaseModel, conint


class A(BaseModel):
    tag: Literal["a"]


class B(BaseModel):
    tag: Literal["b"]
    x: conint(ge=0, le=1)


class Model(BaseModel):
    value: Union[A, B]

Model(value={"tag": "b", "x": 2}) # this will fail with "tag must be 'a'" and "x" must be LE 1

I would expect the error to be just "'x' needs to be LE 1", but the error also complains that "'tag' must be 'a'".

This is somewhat related to #619