Add 400 response on MultiParser exceptions

[Kludex]

• Closes Send 400 on missing boundary #1588
• Closes multipart/form-data subpart without name causes internal error #1303

Let's call this solution 1.

Why I don't like solution 1:

• It adds a new exception to the ExceptionMiddleware, which is weird.

Possible solution 2:

• Use HTTPException with import inside the MultiPartParser to avoid circular import.

Possible solution 3:

• Create an http_exception.py file to hold only HTTPException, and import it on exceptions.py and formparsers.py.
• This solution avoids the antipatterns from previous solutions.

I'm exposing this draft as a self note. Solution 3 looks like the best one.

[Kludex]

Solution 2 and 3 are problematic because we need the information that we are running on a Starlette application or not (so we either raise HTTPException or return a PlainTextResponse), and we can't retrieve that there. 😞 See #1601 for code example.

Possible solution 4:

• Add an exception handler on the application level to MultiParserException.

Solution 4 and 1 are similar...

EDIT: And now I realize that solution 4 and 1 have the same problem as solution 2 and 3. 🥲
EDIT2: Based on this information, I think we need to raise a clear exception on formparser i.e. except TypeError and raise a custom error e.g. MultiPartException with a clear message, and catch that exception on Starlette itself to return a 400. This will have the following behavior:

• On app = Request(scope, receive, send); await request.form() usage, it will raise MultiPartException.
• On app = Starlette() and further usage of .form(), it will return a response with 400.

[Kludex]

Close this in favor of #1617, but the monologue here is important for that one.