add error checking and messages to 'target modules show-unwind' command

git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@189058 91177308-0d34-0410-b5e6-96231b3b80d8
A clang::Type::Decayed type to kill compile warning

git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@189061 91177308-0d34-0410-b5e6-96231b3b80d8