Refuse to run updater if memory_limit is below requirements

[joshtrichards]

See #505, #316, etc.

Since it will fail anyhow, we should simply refuse to run the updater at all if the memory_limit is too low. This is particularly a problem in web-mode.

The message given to the operator should be something like "Too low, either fix the memory_limit to meet official system requirements or update manually. (Tip: the -d memory_limit=xxx option can be used at the command line as quick one-off solution)."