Overload/subtype incompatibility should say why it's incompatible, especially related to default(optional) parameters

```py
from typing import overload

@overload
def foo(a: int=...): ...
@overload
def foo(a: str): ...
def foo(a: object):
print(a)
```
```
test.py:7:1: error: Overloaded function implementation does not accept all possible arguments of signature 1 [misc]
```
ummm, why?

what about:
```
test.py:7:1: error: Overloaded function implementation does not accept all possible arguments of signature 1 [misc]
test.py:7:1: note: parameter "a" is incompatible (it has a default value in the overload)
test.py:7:1: note: "foo()" is "type safe", but will result in a runtime error
```

Reproduce the overload example in the issue with basedmypy and inspect the implementation-compatibility diagnostic path. Compare the current error with the requested explanation for the default parameter, then add or update focused diagnostic coverage so the reported incompatibility identifies why it occurs.

Written by the indexing model from the issue text.