[Object/ELF] - Make getSymbol() return Error.

That is consistent with other methods around
and helps to handle error on a caller side.

Differential revision: https://reviews.llvm.org/D26247

llvm-svn: 285886
diff --git a/llvm/lib/Object/Error.cpp b/llvm/lib/Object/Error.cpp
index c1dfe67..578da22 100644
--- a/llvm/lib/Object/Error.cpp
+++ b/llvm/lib/Object/Error.cpp
@@ -50,6 +50,8 @@
     return "Invalid section index";
   case object_error::bitcode_section_not_found:
     return "Bitcode section not found in object file";
+  case object_error::invalid_symbol_index:
+    return "Invalid symbol index";
   }
   llvm_unreachable("An enumerator of object_error does not have a message "
                    "defined.");