Fix error message on failed target attach being discarded

[iridinite]

If the debugger fails to attach to the specified target, such as due to a permission error, then the error message is accidentally discarded. We create an empty SBStream on the stack, write nothing to it, and then use its (empty) contents as error message.

Instead, this PR removes the empty stream and instead passes the actual error message (as generated by lldb) to the formatted output string, so that it properly shows up in the MI output.

[sunshaoce]

Nice! Could you also add a test to ensure the error message isn’t empty?

[iridinite]

@sunshaoce Hi, I don't seem to be able to run the tests on my dev machine:

CMake Warning at CMakeLists.txt:67 (message):
  Not supported on Windows due to a bug:
  https://gitlab.kitware.com/cmake/cmake/-/issues/22659.  Testing will be
  disabled.

nor do I see source for tests other than a couple for the String class. I'm not sure how to proceed. Could you help me out?

[sunshaoce]

Lack of testing has always been our problem. I haven’t thought of a good solution for this, so let’s merge it first.