Improve warning for UseSHM failures

When using shared memory segements (UseSHM) to get large pages there are typically to types of failures you get when trying to do a "mapping". It either fails with ENOMEM or EPERM:
* ENOMEM will occur when there are no more large pages can be allocated.
* EPERM will occur if the process is not privileged and you have reached the "max locked memory" limit (ulimit -l)

It would be helpful for the user to better indicate this and instead of just printing the errno, give a better error message.

Some good man pages to refer to:
> man shmget - Info on all errors
> man setrlimit - Info on locked mem limit