guix build: Nicely report unbound variables with hints.

* guix/ui.scm (print-unbound-variable-error): Add "error:" to the
message.
(report-unbound-variable-error): New procedure, with code formerly in
'report-load-error'.
(report-load-error): Use it.
(call-with-unbound-variable-handling): New procedure.
(with-unbound-variable-handling): New macro.
* guix/scripts/build.scm (options->derivations): Wrap body in
'with-unbound-variable-handling'.
* tests/guix-build.sh (GUIX_PACKAGE_PATH): Add test.

tests/guix-build.sh

@@ -1,5 +1,5 @@
 # GNU Guix --- Functional package management for GNU
-# Copyright © 2012, 2013, 2014, 2016, 2017 Ludovic Courtès <ludo@gnu.org>
+# Copyright © 2012, 2013, 2014, 2016, 2017, 2018 Ludovic Courtès <ludo@gnu.org>
 #
 # This file is part of GNU Guix.
 #
@@ -138,6 +138,25 @@ test `guix build -d --sources=transitive foo \
       | grep -e 'foo\.tar\.gz' -e 'bar\.tar\.gz' -e 'bar\.dat' \
       | wc -l` -eq 3
 
+
+# Unbound variables.
+cat > "$module_dir/foo.scm"<<EOF
+(define-module (foo)
+  #:use-module (guix tests)
+  #:use-module (guix build-system trivial))
+
+(define-public foo
+  (dummy-package "package-with-something-wrong"
+    (build-system trivial-build-system)
+    (inputs (quasiquote (("sed" ,sed))))))  ;unbound variable
+EOF
+
+if guix build package-with-something-wrong -n; then false; else true; fi
+guix build package-with-something-wrong -n 2> "$module_dir/err" || true
+grep "unbound" "$module_dir/err"		     # actual error
+grep "forget.*(gnu packages base)" "$module_dir/err" # hint
+rm -f "$module_dir"/*
+
 # Should all return valid log files.
 drv="`guix build -d -e '(@@ (gnu packages bootstrap) %bootstrap-guile)'`"
 out="`guix build -e '(@@ (gnu packages bootstrap) %bootstrap-guile)'`"