Protect Flow from exception during request processing

[tchaton]

What does this PR do?

This PR adds a try / catch mechanism around request processing on the flow to avoid failure points coming from the Rest API.

Furthermore, a user can return the expection status code he wishes using fastapi HTTPException.

from fastapi import HTTPException

class FlowAPI(LightningFlow):
    def __init__(self):
        super().__init__()
        self.counter = 0

    def run(self):
         pass

    def request(self, ...):
        self.counter += 1
        if config.index % 5 == 0:
            raise HTTPException(status_code=400, detail="HERE")
        return OutputRequestModel(name=config.name, counter=self.counter)

    def configure_api(self):
        return [Post("/api/v1/request", self.request)]

Note: The state isn't restored if a failure happens. This is left for another PR.

Fixes #<issue_number>

Does your PR introduce any breaking changes? If yes, please list them.

Before submitting

• Was this discussed/approved via a GitHub issue? (not for typos and docs)
• Did you read the contributor guideline, Pull Request section?
• Did you make sure your PR does only one thing, instead of bundling different changes together?
• Did you make sure to update the documentation with your changes? (if necessary)
• Did you write any new necessary tests? (not for typos and docs)
• Did you verify new and existing tests pass locally with your changes?
• Did you list all the breaking changes introduced by this pull request?
• Did you update the CHANGELOG? (not for typos, docs, test updates, or minor internal changes/refactors)

PR review

Anyone in the community is welcome to review the PR.
Before you start reviewing, make sure you have read the review guidelines. In short, see the following bullet-list:

• Is this pull request ready for review? (if not, please submit in draft mode)
• Check that all items from Before submitting are resolved
• Make sure the title is self-explanatory and the description concisely explains the PR
• Add labels and milestones (and optionally projects) to the PR so it can be classified

Did you have fun?

Make sure you had fun coding 🙃

cc @Borda

[ethanwharris]

LGTM, small question