ugtrain: handle ptrace() errors gracefully

In Warzone 2100 we've noticed issues when reading memory in
challenge games with a lot of AI players. With this patch it works
without an issue.

It is also better for running 'scanmem' in parallel. They also
don't exit upon a ptrace() error. So do it the same way.
ugtrain: update victim memory r/w error messages

We want to know at which memory address couldn't be read or written
to. Having the term 'PTRACE' in it isn't good for multi-platform
support. So change that.
ugtrain: read multiple pages from the dynmem FIFO

In the second single-player challenge in Warzone 2100 there are
806 Structure objects which are created at once. This is too much
for a single page read and leads to instability and victim memory
r/w errors. So read until the FIFO buffer is empty.