Prevent TestLldbGdbServer shutdown cleanup failures from failing the test.

Adds try/except blocks around clean-up code. Prevents a race between gdb
remote kill command reception by llgs (which leads llgs to shut down)
and the pexpect server kill (which can fail if the kill command handling
completes first). Warnings are emitted on the logger for any clean-up
code that fails.

llvm-svn: 207273