report out-of-memory usefully

[zookozcash]

A very frequent issue among our users currently is that they run out of RAM, get a mysterious error message like St13runtime_exception, and start asking on our community channels for help.

To close this ticket, make it so at startup it tries to allocate 8 GB of RAM (or whatever), and write to it, and if it can't, then it emits a useful error message that explains to the user "You appear to have too little RAM to run this. We need at least 8 GB of RAM."

Then free the RAM. (Don't worry, the operating system won't let any other processes use it anyway, and later in this process when it tries to allocate that much again, it will work.)

[zookozcash]

Related: #1083

[zookozcash]

Example of frustrated user who would be helped by fixing this issue: https://ravibhojwani.wordpress.com/digital-ecomony/

[ioptio]

8GB of RAM? I was able to compile with less as far as I remember.

[ageis]

@ioptio I think this ticket refers to runtime, not build-time.

[ioptio]

Oh... I get a different error message which is documented on Beta Guide when I run out of RAM on run-time: std::bad_alloc

Ok, sorry. I was directed at this topic with reference to compiling memory problems but that outputs a different error.

Confirming that I see the following when my node runs out of memory:
terminate called after throwing an instance of 'std::bad_alloc' what(): std::bad_alloc

[tromer]

Related to #1966, "Reduce memory consumption due to proving key" and #1940, "multi-core proof time & memory measurements".