show more context for spawnp error message

The old error message was

`No such file or directory - posix_spawnp`

That confuses people into thinking that they're missing a dependency called posix_spawnp, when what's actually happened is that posix_spawnp returned ENOENT when trying to execute, um, something.  Ruby locks us into this `strerror + " - " + function` ordering that is opposite the `function + ": " + strerror` order that people are used to, and that `perror` provides.

The new error message just piles more information into the `mesg` field, like so:

`No such file or directory - when spawning 'xxgh-gitmon'`