[lldb][NFCI] Rewrite error-handling code in ProcessGDBRemote::MonitorDebugserverProcess

I'm rewriting this for 2 reasons:
1) Although a 1024 char buffer is probably enough space, the error
   string may grow beyond that and we could lose some information. Using
   StringStream (as we do in the rest of ProcessGDBRemote) seems like a
   sensible solution.
2) I am planning on changing `UnixSignals::GetSignalAsCString`,
   rewriting things with `llvm::formatv` will make that process easier.

Differential Revision: https://reviews.llvm.org/D158017