Provide meaningful error message for multi-cradle hie.yaml

[alanz]

Us immigrants from hie are accustomed to having multi-cradle just work, so tend to have hie.yaml config files set up for multi-component use.

Running a bare ghcide on one of these does not give an obvious error message. e.g.

$ ghcide
ghcide version: 0.1.0 (GHC: 8.6.5) (PATH: /home/alanz/.cabal/store/ghc-8.6.5/ghcide-0.1.0-ee324fcd43ac182a56f9ab18d8ff0a02e26330a448eb057350d92e6d7b3141f5/bin/ghcide)
Ghcide setup tester in /home/alanz/tmp/hie-t2.
Report bugs at https://github.com/digital-asset/ghcide/issues

Step 1/6: Finding files to test in /home/alanz/tmp/hie-t2
Found 3 files

Step 2/6: Looking for hie.yaml files that control setup
Found 1 cradle

Step 3/6, Cradle 1/1: Loading /home/alanz/tmp/hie-t2/hie.yaml

Step 4/6, Cradle 1/1: Loading GHC Session
ghcide: CradleError ExitSuccess ["Multi Cradle: No prefixes matched","pwd: /home/alanz/tmp/hie-t2","filepath: /home/alanz/tmp/hie-t2","prefixes:","(\"src/B.hs\",Cabal {component = Just \"lib:cabalnewbuild-test\"})","(\"src/Format\",Cabal {component = Just \"lib:cabalnewbuild-test\"})","(\"A.hs\",Cabal {component = Just \"cabalnewbuild-test:exe:test\"})"]

It says ExitSuccess, and then that no prefixes matched for the Multi Cradle. This does not imply that Mult Cradle is not supported.

[jneira]

As multi-cradle is already supported in ghcide this is not reproducible anymore