When gdbserial can not find debugserver or lldb-server the error
message is always the same and it complains about lldb-server not being
found.
This is fine on linux (where the backend is unnecessary) but incomplete
on macOS (where the backend is actually used).
Make the error message clearer so that users who do not bother reading
install instructions are not confused.
The authorization prompt on macOS can take a long time to be
acknowledged by the user, we should keep waiting for a connection as
long as the debugserver instance we launched remains alive.
