gnu: Improve error reporting of the use-.*modules macros.

Suggested by Julien Lepiller and myglc2
at <https://lists.gnu.org/archive/html/guix-devel/2017-11/msg00106.html>.

* gnu.scm (%try-use-modules): New procedure.
(package-module-hint, service-module-hint): New procedures.
(try-use-modules): New macro.
(use-package-modules, use-service-modules, use-system-modules): Use it.
* tests/guix-system.sh: Test it.

tests/guix-system.sh

@@ -68,6 +68,34 @@ else
 fi
 
 
+# Reporting of module not found errors.
+
+cat > "$tmpfile" <<EOF
+;; Line 1.
+(use-modules (gnu))
+  (use-service-modules openssh)
+EOF
+
+if guix system build "$tmpfile" -n 2> "$errorfile"
+then false
+else
+    grep "$tmpfile:3:2: .*module .*openssh.*not found" "$errorfile"
+    grep "Try.*use-service-modules ssh" "$errorfile"
+fi
+
+cat > "$tmpfile" <<EOF
+;; Line 1.
+(use-modules (gnu))
+  (use-package-modules qemu)
+EOF
+
+if guix system build "$tmpfile" -n 2> "$errorfile"
+then false
+else
+    grep "$tmpfile:3:2: .*module .*qemu.*not found" "$errorfile"
+    grep "Try.*use-package-modules virtualization" "$errorfile"
+fi
+
 # Reporting of unbound variables.
 
 cat > "$tmpfile" <<EOF