[ty] Improve diagnostics for invalid exceptions (#21475)

## Summary

Not a high-priority task... but it _is_ a weekend :P

This PR improves our diagnostics for invalid exceptions. Specifically:
- We now give a special-cased ``help: Did you mean
`NotImplementedError`` subdiagnostic for `except NotImplemented`, `raise
NotImplemented` and `raise <EXCEPTION> from NotImplemented`
- If the user catches a tuple of exceptions (`except (foo, bar, baz):`)
and multiple elements in the tuple are invalid, we now collect these
into a single diagnostic rather than emitting a separate diagnostic for
each tuple element
- The explanation of why the `except`/`raise` was invalid ("must be a
`BaseException` instance or `BaseException` subclass", etc.) is
relegated to a subdiagnostic. This makes the top-level diagnostic
summary much more concise.

## Test Plan

Lots of snapshots. And here's some screenshots:

<details>
<summary>Screenshots</summary>

<img width="1770" height="1520" alt="image"
src="https://github.com/user-attachments/assets/7f27fd61-c74d-4ddf-ad97-ea4fd24d06fd"
/>

<img width="1916" height="1392" alt="image"
src="https://github.com/user-attachments/assets/83e5027c-8798-48a6-a0ec-1babfc134000"
/>

<img width="1696" height="588" alt="image"
src="https://github.com/user-attachments/assets/1bc16048-6eb4-4dfa-9ace-dd271074530f"
/>

</details>

crates/ty_python_semantic/resources/mdtest/exception/basic.md

@@ -116,17 +116,18 @@ def silence2[T: (
 
 ## Invalid exception handlers
 
+<!-- snapshot-diagnostics -->
+
 ```py
 try:
     pass
-# error: [invalid-exception-caught] "Cannot catch object of type `Literal[3]` in an exception handler (must be a `BaseException` subclass or a tuple of `BaseException` subclasses)"
+# error: [invalid-exception-caught]
 except 3 as e:
     reveal_type(e)  # revealed: Unknown
 
 try:
     pass
-# error: [invalid-exception-caught] "Cannot catch object of type `Literal["foo"]` in an exception handler (must be a `BaseException` subclass or a tuple of `BaseException` subclasses)"
-# error: [invalid-exception-caught] "Cannot catch object of type `Literal[b"bar"]` in an exception handler (must be a `BaseException` subclass or a tuple of `BaseException` subclasses)"
+# error: [invalid-exception-caught]
 except (ValueError, OSError, "foo", b"bar") as e:
     reveal_type(e)  # revealed: ValueError | OSError | Unknown
 
@@ -278,3 +279,20 @@ def f(x: type[Exception]):
     # error: [possibly-unresolved-reference]
     reveal_type(e)  # revealed: None
 ```
+
+## Special-cased diagnostics for `NotImplemented`
+
+<!-- snapshot-diagnostics -->
+
+```py
+try:
+    # error: [invalid-raise]
+    # error: [invalid-raise]
+    raise NotImplemented from NotImplemented
+# error: [invalid-exception-caught]
+except NotImplemented:
+    pass
+# error: [invalid-exception-caught]
+except (TypeError, NotImplemented):
+    pass
+```