Slight edit to error msg. Updated unit test.

Author: aadanen

clang/lib/Interpreter/Interpreter.cpp

@@ -758,7 +758,7 @@ llvm::Error Interpreter::Undo(unsigned N) {
                                                std::error_code());
   } else if (N > getEffectivePTUSize()) {
     return llvm::make_error<llvm::StringError>(
-        llvm::formatv("Wanted to undo {0} inputs, only have {1}.",
+        llvm::formatv("Operation failed. Wanted to undo {0} inputs, only have {1}.",
             N, getEffectivePTUSize()),
         std::error_code());
   }

clang/unittests/Interpreter/InterpreterTest.cpp

@@ -163,7 +163,7 @@ TEST_F(InterpreterTest, UndoCommand) {
   auto Err2 = Interp->Parse("int foo = 42;");
   EXPECT_TRUE(!!Err2);
   auto Err3 = Interp->Undo(2);
-  EXPECT_EQ("Operation failed. No input left to undo",
+  EXPECT_EQ("Operation failed. Wanted to undo 2 inputs but only have 1.",
             llvm::toString(std::move(Err3)));
 
   // Succeed to undo.