Fix a rare bug in StartDaemonChild and GenericMain

I've seen cases where the result from str(sys.exc_info()[1]) is ""; this
breaks the error reporting as the parent relies on non-empty error
messages to properly detect child status (otherwise it will try to read
the pid and fail, so on).

While this was always in case of asserts, we need to ensure this doesn't
happen. Therefore we abstract this functionality (writing the error
message) and ensure we write a non-empty string in the new function.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>