llgs: Send "rich" errors in response to vAttach packets

llgs: Send "rich" errors in response to vAttach packets

There are plenty of ways attaching can go wrong. Having the server
report the exact error means we can give better feedback to the user.
(This patch does not do the second part, it only makes sure the
information is sent from the server.)

Triggering all possible error conditions in a test would prove
challenging, but there is one error that is very easy to reproduce
(attempting to attach while debugging), so I write a test based on that.

The test immediately exposed a bug where the m_send_error_strings field
was being used uninitialized (so it was sometimes true from the get-go),
so I fix that as well.

llvm-svn: 329803