thread state coordinator: requesting resume now signals error appropriately.

Added tests to verify that the coordinator signals an error if
the given thread to resume is unknown, and if the thread is through to
be running already.

Modified resume handling code to match tests.


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@218872 91177308-0d34-0410-b5e6-96231b3b80d8