Fix the servor 500 error and show error message to client

[unkwnownGriot]

create a bad_request function to handle errors during data processing

put the vectorstore setting , index loading, prompt template creating and answer fetching inside a `try/except block and returning the exception occured to the client

refactor the form submitting function to show an alert when an exception is raised from the backend

[dartpain]

Great Job! thank you!