Bug 215049

Several developers have reported build failures due to the output of generate-message-receiver.py including a 4K chunk of NULs at the start of the generated files. It's not apparent if this is a script issue, a Python issue, an OS issue, a hardware issue, or what. But in order to make this less of an issue, update generate-message-receiver.py as follows:

- When files are written, check for the corruption. If it occurs, retry up to five times. Write a message to stderr if corruption occurs in order to help corral the issue. After five times, give up.
- Add support for the WK_SKIP_RETRY_ON_CORRUPTION environment variable. If set to non-zero/empty, then give up without retrying. By not retrying, the adventurous developer may be able to examine the conditions that led to the corruption and figure out what happened.