Exceptions inheriting from ValidationError raised in dec_hook implicitly wrapped in another ValidationError post 0.21.0 #1012

Description

Pre 0.21.0, this would raise SomeError directly, now it raises msgspec.ValidationError with SomeError as a cause:

import msgspec

class SomeError(msgspec.ValidationError):
    pass

class Foo:
    pass

def dec_foo(tp, val):
    raise SomeError()

try:
    msgspec.convert("", type=Foo, dec_hook=dec_foo)
except Exception as exc:
    print(type(exc))

I believe this was an unintended side-effect of #790.

I'm not sure if this is desirable or not. It is however a breaking change