ui: 'warn-about-load-error' provides hints for unbound variables.

* guix/ui.scm (warn-about-load-error): Add 'unbound-variable' clause.
* tests/guix-build.sh: Add test.

guix/ui.scm

@@ -392,6 +392,8 @@ exiting.  ARGS is the list of arguments received by the 'throw' handler."
     (('syntax-error proc message properties form . rest)
      (let ((loc (source-properties->location properties)))
        (warning loc (G_ "~a~%") message)))
+    (('unbound-variable _ ...)
+     (report-unbound-variable-error args))
     (('srfi-34 obj)
      (if (message-condition? obj)
          (warning (G_ "failed to load '~a': ~a~%")

tests/guix-build.sh

@@ -146,7 +146,7 @@ test `guix build -d --sources=transitive foo \
       | wc -l` -eq 3
 
 
-# Unbound variables.
+# Unbound variable in thunked field.
 cat > "$module_dir/foo.scm" <<EOF
 (define-module (foo)
   #:use-module (guix tests)
@@ -162,6 +162,21 @@ if guix build package-with-something-wrong -n; then false; else true; fi
 guix build package-with-something-wrong -n 2> "$module_dir/err" || true
 grep "unbound" "$module_dir/err"		     # actual error
 grep "forget.*(gnu packages base)" "$module_dir/err" # hint
+
+# Unbound variable at the top level.
+cat > "$module_dir/foo.scm" <<EOF
+(define-module (foo)
+  #:use-module (guix tests))
+
+(define-public foo
+  (dummy-package "package-with-something-wrong"
+    (build-system gnu-build-system)))      ;unbound variable
+EOF
+
+guix build sed -n 2> "$module_dir/err"
+grep "unbound" "$module_dir/err"		     # actual error
+grep "forget.*(guix build-system gnu)" "$module_dir/err" # hint
+
 rm -f "$module_dir"/*
 
 # Wrong 'define-module' clause reported by 'warn-about-load-error'.