Report more precise error message when attach fails

Summary:
If the remote stub sends a specific error message instead of just a E??
code, we can use this to display a more informative error message
instead of just the generic "unable to attach" message.

I write a test for this using the SB API.
On the console this will show up like:
(lldb) process attach ...
error: attach failed: <STUB-MESSAGE>

if the stub supports error messages, or:
error: attach failed: Error ??

if it doesn't.

Reviewers: jingham, JDevlieghere

Subscribers: lldb-commits

Differential Revision: https://reviews.llvm.org/D45573

llvm-svn: 330247