Give readProcess' more sensible error messages.

Also a bunch of refactoring for GhcModError

test/FindSpec.hs

@@ -2,10 +2,11 @@ module FindSpec where
 
 import Language.Haskell.GhcMod.Find
 import Test.Hspec
+import TestUtils
 
 spec :: Spec
 spec = do
     describe "db <- loadSymbolDb" $ do
         it "lookupSymbol' db \"head\"  contains at least `Data.List'" $ do
-            db <- loadSymbolDb
+            db <- runD loadSymbolDb
             lookupSym "head" db `shouldContain` ["Data.List"]