[LLGS] Log more precise errors during inferior launch

Summary:
We are seeing infrequent failures to launch the inferior process on android. The failing call
seems to be execve(). This adds more logging to see the actual error reported by the call.

Reviewers: tberghammer

Subscribers: tberghammer, lldb-commits, danalbert

Differential Revision: http://reviews.llvm.org/D22039

llvm-svn: 274624