Enhance error message

[yellowbean]

I've already include servant-errors

Hastructure/app/Main.hs

Line 453 in d813a25

$ errorMwDefJson

But seems that if error was trigger by here:

Hastructure/src/Cashflow.hs

Line 511 in d813a25

mflowInterest _ = error "not supported"

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]

haskell-servant/servant#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

[yellowbean]

Now response of each call shall be Either Left Right ,the Left represent error message .

a5c6f52