sanity.bbclass: add more information to error message about TUNE_PKGARCH missing...

sanity.bbclass: add more information to error message about TUNE_PKGARCH missing in PACKAGE_ARCHS

* sometimes it's hard to see what's wrong here
* error message:
  Error, the PACKAGE_ARCHS variable does not contain TUNE_PKGARCH (cortexa7t2hfhf-vfp-vfpv4-neon).
  doesn't help much to understand what exactly went wrong and where

(From OE-Core rev: 4bedd149964b3b594d7b1a233a9cc436925288e9)

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>