add @[fail] modifier to declarations

- `@[fail] def foo = ...` succeeds if `foo` has some error.
- `@[fail "scope"] def foo = ...` succeeds if `foo` has some error
  containing the word "scope" somewhere
- `@[fail] namespace foo { }` works too and the error must be anywhere
  in the namespace

lib/Quox/Parser/FromParser/Error.idr

@@ -30,6 +30,8 @@ data Error =
   | DisplacedBoundVar Loc PName
   | WrapTypeError TypeError
   | LoadError Loc FilePath FileError
+  | ExpectedFail Loc
+  | WrongFail String Error Loc
   | WrapParseError String ParseError
 
 
@@ -106,5 +108,13 @@ parameters (showContext : Bool)
           "couldn't load file" <++> text file,
           text $ show err]
 
+  prettyError (ExpectedFail loc) = pure $
+    sep [!(prettyLoc loc), "expected error"]
+
+  prettyError (WrongFail str err loc) = pure $
+    sep [!(prettyLoc loc),
+         "wrong error, expected to match", !(hl Tag $ text "\"\{str}\""),
+         "but got", !(prettyError err)]
+
   prettyError (WrapParseError file err) =
     prettyParseError file err