[SourceKit] Fail requests when an error occurs

Previously, requests would fail silently by returning an empty struct
in the response.

With this change, responses will properly report fail with the internal
error.

unittests/SourceKit/SwiftLang/CursorInfoTest.cpp

@@ -90,6 +90,8 @@ public:
 };
 
 struct TestCursorInfo {
+  // Empty if no error.
+  std::string Error;
   std::string Name;
   std::string Typename;
   std::string Filename;
@@ -146,7 +148,8 @@ public:
 
     TestCursorInfo TestInfo;
     getLang().getCursorInfo(DocName, Offset, 0, false, false, Args,
-      [&](const CursorInfoData &Info) {
+      [&](const CursorInfoData &Info, StringRef Error) {
+        TestInfo.Error = Error;
         TestInfo.Name = Info.Name;
         TestInfo.Typename = Info.TypeName;
         TestInfo.Filename = Info.Filename;