python: enforce the existence of files in the filespecs

Seems that this allows some goofs, because some files
silently do not get copied and the build succeeds, even though
it shouldn't.

Signed-off-by: Alexandru Ardelean <ardeleanalex@gmail.com>

lang/python/files/python-package.mk

 		IFS='|'; \
 		while read fop fspec fperm; do \
 		  if [ "$$$$$$$$fop" = "+" ]; then \
+			if [ ! -e "$(PKG_INSTALL_DIR)$$$$$$$$fspec" ]; then \
+			  echo "File not found '$(PKG_INSTALL_DIR)$$$$$$$$fspec'"; \
+			  exit 1; \
+			fi; \
 			dpath=`dirname "$$$$$$$$fspec"`; \
 			if [ -n "$$$$$$$$fperm" ]; then \
 			  dperm="-m$$$$$$$$fperm"; \