Enhance error message

[yellowbean]

I've already include servant-errors

https://github.com/yellowbean/Hastructure/blob/d813a25fc82b13538d0e1ba4a032d33de9ffffcb/app/Main.hs#L453

But seems that if error was trigger by here: https://github.com/yellowbean/Hastructure/blob/d813a25fc82b13538d0e1ba4a032d33de9ffffcb/src/Cashflow.hs#L511

the error "xxxxx" isn't convert into a json response like :

{"status":xxx
 "message":"xxxxxx"}

from the python/request , a remoteDisconnected was raised.

[yellowbean]

Research by far:

• Servant Checked-Exception -> it has to annotate all the function with envelop ,which will significantly change the code base.
• Servant Exception -> Failed in example case in later. but it's good to have on validation on the API endpoint level

[yellowbean]

https://github.com/haskell-servant/servant/issues/1192

seems the issue hasn't been resolved.

[yellowbean]

The last resort will be create a new api endpoints with response type include a error message