Make the lldbtest tear down routine a little less error prone
- replace "catch-all" except clause with one that specifically catches what pexpect throws
- handle case where child is already terminated (or is terminating) by the time tear-down is run



git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@175844 91177308-0d34-0410-b5e6-96231b3b80d8