getrandom() syscall failure warning should be a notice and worded better

The logging improvement for error handling of the getrandom() syscall in legacy/trac#24500 (moved) could be further improved:

• The log level should possibly be NOTICE rather than WARN.
• The log message should mention that tor will fall back to alternative sources of randomness.
• Maybe we want to mention header/kernel version mismatches as a specific common reason for this issue.

Yes, some of the fallbacks are somewhat dangerous, like /dev/urandom might not be seeded yet. etc,.