[NFC] Fix error handling documentation

[mstojanovic]

Summary

The default constructor can't be used since rL286561.

[lhames]

LGTM. Thanks Miloš!

llvm/docs/ProgrammersManual.rst

 this, use the named constructor idiom and return an ``Expected<T>``:
 
 .. code-block:: c++
 
   class Foo {
   public:
 
     static Expected<Foo> Create(Resource R1, Resource R2) {
-      Error Err;
+      Error Err = Error::success();
       Foo F(R1, R2, Err);
       if (Err)
         return std::move(Err);
       return std::move(F);
     }
 
   private:
 

 
 The archive walking examples above retrieve archive members by index, however
 this requires considerable boiler-plate for iteration and error checking. We can
 clean this up by using the "fallible iterator" pattern, which supports the
 following natural iteration idiom for fallible containers like Archive:
 
 .. code-block:: c++
 
-  Error Err;
+  Error Err = Error::success();
   for (auto &Child : Ar->children(Err)) {
     // Use Child - only enter the loop when it's valid
 
     // Allow early exit from the loop body, since we know that Err is success
     // when we're inside the loop.
     if (BailOutOn(Child))
       return;
 

llvm/include/llvm/Object/MachO.h

public:
   DataRefImpl getRawDataRefImpl() const;
   const ObjectFile *getObjectFile() const;
 };
 using dice_iterator = content_iterator<DiceRef>;
 
 /// ExportEntry encapsulates the current-state-of-the-walk used when doing a
 /// non-recursive walk of the trie data structure.  This allows you to iterate
 /// across all exported symbols using:
-///      Error Err;
+///      Error Err = Error::success();
 ///      for (const llvm::object::ExportEntry &AnExport : Obj->exports(&Err)) {
 ///      }
 ///      if (Err) { report error ...
 class ExportEntry {
 public:
   ExportEntry(Error *Err, const MachOObjectFile *O, ArrayRef<uint8_t> Trie);
 
   StringRef name() const;
private:
 
   SmallVector<SectionInfo, 32> Sections;
   int32_t MaxSegIndex;
 };
 
 /// MachORebaseEntry encapsulates the current state in the decompression of
 /// rebasing opcodes. This allows you to iterate through the compressed table of
 /// rebasing using:
-///    Error Err;
+///    Error Err = Error::success();
 ///    for (const llvm::object::MachORebaseEntry &Entry : Obj->rebaseTable(&Err)) {
 ///    }
 ///    if (Err) { report error ...
 class MachORebaseEntry {
 public:
   MachORebaseEntry(Error *Err, const MachOObjectFile *O,
                    ArrayRef<uint8_t> opcodes, bool is64Bit);
 
private:
   uint8_t  PointerSize;
   bool     Done = false;
 };
 using rebase_iterator = content_iterator<MachORebaseEntry>;
 
 /// MachOBindEntry encapsulates the current state in the decompression of
 /// binding opcodes. This allows you to iterate through the compressed table of
 /// bindings using:
-///    Error Err;
+///    Error Err = Error::success();
 ///    for (const llvm::object::MachOBindEntry &Entry : Obj->bindTable(&Err)) {
 ///    }
 ///    if (Err) { report error ...
 class MachOBindEntry {
 public:
   enum class Kind { Regular, Lazy, Weak };
 
   MachOBindEntry(Error *Err, const MachOObjectFile *O,