ProtoLogTool: Fix thread pool leak on code generation exception

Fixes an issue where the thread pool leaks when code generation
throws an exception (for example because of an invalid log string).

When the thread pool leaks, System.exit() no longer works, because
the JVM waits for all threads to exit - which never happens for the
leaked thread pool.

Test: `ProtoLog.w(LOG_GROUP, new String())` -> make droid should not hang
Change-Id: Ibdae0f1e68441c6fe004398d146f496503ccc4cd