getsafe: Better explain why the build is stopped

Improve user experience.

Thanks to Jesse Rehmer for the suggestion.

site/getsafe.sh

@@ -27,14 +27,21 @@ if [ -f SCCS/s.${DEST} ]; then
     exec sccs get -s ${DEST}
 fi
 
-##  File exist locally?
+##  Does the file exist locally?
 if [ -f ${DEST} ]; then
     cmp ${SRC} ${DEST}
     if [ $? -eq 0 ]; then
         touch ${DEST}
         exit 0
     fi
-    echo "${SRC} has changed; please update ${DEST}"
+    diff -u ${SRC} ${DEST}
+    echo "${SRC} has changed since the last build."
+    echo "Please ensure the above changes wouldn't be worth adding"
+    echo "to your ${DEST} production file."
+    echo "  Once taken into account or if you don't mind the proposed changes,"
+    echo "  run \"rm site/${DEST}\" and continue the build."
+    echo "  Note that you can silent all these warnings by running"
+    echo '  "cd site && make distclean".'
     exit 1
 fi