Clarify string coercion errors

Author: layus

src/Nix/Convert.hs

@@ -205,7 +205,7 @@ instance ( Convertible e t f m
           (M.lookup "outPath" s)
       _ -> stub
 
-  fromValue = fromMayToValue $ TString NoContext
+  fromValue = fromMayToValue $ TString HasContext
 
 instance Convertible e t f m
   => FromValue ByteString m (NValue' t f m (NValue t f m)) where

src/Nix/Value.hs

@@ -754,8 +754,8 @@ describeValue =
     TFloat             -> "a float"
     TBool              -> "a boolean"
     TNull              -> "a null"
-    TString NoContext  -> "a string"
-    TString HasContext -> "a string with context"
+    TString NoContext  -> "a string with no context"
+    TString HasContext -> "a string"
     TList              -> "a list"
     TSet               -> "an attr set"
     TClosure           -> "a function"