Fix a thinko in the case where we return a launch error with no error
string.  Return "<unknown error>" rather than the empty launch error...

<rdar://problem/20026469>

llvm-svn: 231287
diff --git a/lldb/tools/debugserver/source/debugserver.cpp b/lldb/tools/debugserver/source/debugserver.cpp
index c46d63d..2c27511 100644
--- a/lldb/tools/debugserver/source/debugserver.cpp
+++ b/lldb/tools/debugserver/source/debugserver.cpp
@@ -255,7 +255,7 @@
     {
         DNBLogThreaded ("%s DNBProcessLaunch() failed to launch process, unknown failure", __FUNCTION__);
         ctx.LaunchStatus().SetError(-1, DNBError::Generic);
-        ctx.LaunchStatus().SetErrorString(launch_err_str);
+        ctx.LaunchStatus().SetErrorString("<unknown failure>");
     }
     else
     {