Improve missing brittany format provider error message (#269)

[willbush]

I have tested this error message with cabal. By setting haskell-language-server/haskell-language-server.cabal:

flag agpl
  description: Enable AGPL dependencies
  default:     True
  manual:      True

to

flag agpl
  description: Enable AGPL dependencies
  default:     False
  manual:      True

using cabal build exe:haskell-language-server

And after setting my editor to use cabal exec which haskell-language-server and the Brittany formatter. I can see the error message show up correctly.

I thought about adding a test to haskell-language-server/test/functional/Format.hs for when AGPL is false, but wasn't sure if it would ever be run. Do CI tests with this flag on or off?

Please let me know if style should be changed or if the error message is too verbose.

[lukel97]

Sorry for the delay, this is a good idea. Thanks!

[jneira]

@willbush wow, many thanks and sorry for the delay, merging!