[FIX] Make ffi.Error picklable (#709)

## Motivation

`Error.py_error()` attaches the FFI error object to the Python exception
it raises:

```python
py_error.__tvm_ffi_error__ = self
```

`BaseException.__reduce__` includes the instance `__dict__`, so pickling
an FFI-originated exception pickles that `Error` through
`CObject.__getstate__` → `ToJSONGraph`. `ffi.Error` has no native
creator and no `__ffi_new__` type attr, so this fails:

```
TypeError: Type `ffi.Error` does not support ToJSONGraph
           (no native creator or __ffi_new__ type attr)
```

Harnesses that ship exceptions across a process boundary lose the
original error and report this instead. It was reported downstream
against a CUDA IMA (error 700), where the replacement message made it
look like tvm-ffi had suppressed a coredump — it had not; the coredump
happens before the error is raised, and the only real defect is that the
exception could not be pickled.

`copy.deepcopy` hits the same path.

## Changes

`ffi.Error` now pickles by value. An error is fully described by its
`(kind, message, backtrace)` strings, so `Error.__reduce__` reconstructs
it through `__init__` rather than the JSON graph. This mirrors `Device`
and `DataType`, which already carry hand-written `__reduce__`
implementations for the same reason.

`extra_context` is intentionally dropped — it may hold arbitrary native
payloads, and preserving it only when it happens to be serializable
would make pickling an error depend on where the error came from.

The attribute stays a live `Error` rather than degrading to `None`,
because `set_last_ffi_error` calls `update_backtrace` on
`__tvm_ffi_error__` when a restored exception propagates back through
the FFI; a `None` there would fail on an unrelated `hasattr` check that
is already in the code.

## Not in scope

An earlier revision of this PR also rewrote the message raised when
pickling types that genuinely cannot round-trip (`ffi.Function`,
`ffi.Module`, `ffi.Tensor`, `ffi.OpaquePyObject`). Per review, that is
dropped here and can be revisited separately — this PR is now only the
`Error` fix. Those types are unchanged and still surface the
serializer's own message.

## Testing

New tests in `tests/python/test_error.py`: exception and bare-`Error`
pickle round trips, `deepcopy`, NULL handle, the `extra_context` drop,
and re-propagating a restored exception through an FFI call (which
exercises `set_last_ffi_error`'s `update_backtrace` on the restored
object).

Full Python suite: 2421 passed, 52 skipped, 2 xfailed. `pre-commit`
clean on all changed files.