The pkg_config helper for premake was not checking for errors. When it
failed to run, either when it is not installed or the queried package is
not found, a cryptic error message would be printed:
"Error: .../xenia/third_party/premake-core/src/base/string.lua:36: attempt to index a nil value (upvalue 's')"
Fix this by checking the return status when calling pkg-config and
printing a descriptive error message.
Alex Messier