Introduce a "failed" value type

In the multithreaded evaluator, it's possible for multiple threads to
wait on the same thunk. If evaluation of the thunk results in an
exception, the waiting threads shouldn't try to re-force the thunk.
Instead, they should rethrow the same exception, without duplicating
any work.

Therefore, there is now a new value type `tFailed` that stores an
std::exception_ptr. If evaluation of a thunk/app results in an
exception, `forceValue()` overwrites the value with a `tFailed`. If
`forceValue()` encounters a `tFailed`, it rethrows the exception. So
you normally never need to check for failed values (since forcing them
causes a rethrow).

src/libexpr/print-ambiguous.cc

@@ -75,6 +75,9 @@ void printAmbiguous(
             str << "«potential infinite recursion»";
         }
         break;
+    case nFailed:
+        str << "«failed»";
+        break;
     case nFunction:
         if (v.isLambda()) {
             str << "<LAMBDA>";