lldb-gdbserver: add support for optional host in connection string and handle other startup errors.

This change addresses the following bug:
http://www.llvm.org/bugs/show_bug.cgi?id=18814


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@202910 91177308-0d34-0410-b5e6-96231b3b80d8