[LTO] Fix error reporting when a file passed to libLTO is invalid or non-existent

This addresses PR26060 where function lto_module_create() could return nullptr
but lto_get_error_message() returned an empty string.

The error() call after LTOModule::createFromFile() in llvm-lto is then removed
because any error from this function should go through the diagnostic handler in
llvm-lto which will exit the program. The error() call was added because this
previously did not happen when the file was non-existent. This is fixed by the
patch. (The situation that llvm-lto reports an error when the input file does
not exist is tested by llvm/tools/llvm-lto/error.ll).

Differential Revision: http://reviews.llvm.org/D16106

llvm-svn: 258298