Properly error when trying to Union

[saulshanabrook]

Add a helpful error message when trying to use a union of two eqsorts as a type.

This should error:

from typing import TypeAlias, Union
from egglog import *

class A(Expr):
    def __init__(self) -> None: ...

class B(Expr):
    def __init__(self) -> None: ...

CustomLike: TypeAlias = Union[A, B]

class C(Expr):
    def __init__(self, some_input: CustomLike) -> None: ...

a = A()
b = B()

C(a) # ok
C(b) # not ok... :(

https://egraphs.zulipchat.com/#narrow/channel/375765-egg.2Fegglog/topic/multiple.20sort.20inputs.20.28via.20Union.29.3F/near/484334683