Make the Error class constructor protected

This is forcing to use Error::success(), which is in a wide majority
of cases a lot more readable.

Differential Revision: https://reviews.llvm.org/D26481

llvm-svn: 286561
diff --git a/llvm/tools/llvm-objdump/llvm-objdump.cpp b/llvm/tools/llvm-objdump/llvm-objdump.cpp
index 3f6ef52..920ad4a 100644
--- a/llvm/tools/llvm-objdump/llvm-objdump.cpp
+++ b/llvm/tools/llvm-objdump/llvm-objdump.cpp
@@ -1969,7 +1969,7 @@
 
 /// @brief Dump each object file in \a a;
 static void DumpArchive(const Archive *a) {
-  Error Err;
+  Error Err = Error::success();
   for (auto &C : a->children(Err)) {
     Expected<std::unique_ptr<Binary>> ChildOrErr = C.getAsBinary();
     if (!ChildOrErr) {