This change surfaces the static Fuzzer callbacks used to handle errors, in anticipation of them being called by a platform-specific IPC library to create cross-process fuzzers.
It also adds a PID parameter to process-specific diagnostic functions used by those callbacks, e.g. PrintStackTrace. Currently, this parameter is always 0, which indicates the local process.
This is change 9 of (at least) 18 for cross-process fuzzing support.