guix-build: Gracefully handle `&package-input-error' conditions.

* guix/packages.scm: Export `package-error?' and `package-input-error?'.
* guix-build.in (guix-build): Catch `&package-input-error' conditions,
  print a human-readable message, and exit.

guix-build.in

@@ -30,11 +30,12 @@ exec ${GUILE-@GUILE@} -L "@guilemoduledir@" -l "$0"    \
   #:use-module (guix store)
   #:use-module (guix derivations)
   #:use-module (guix packages)
-  #:use-module ((guix utils) #:select (%current-system))
+  #:use-module (guix utils)
   #:use-module (ice-9 format)
   #:use-module (ice-9 match)
   #:use-module (srfi srfi-1)
   #:use-module (srfi srfi-26)
+  #:use-module (srfi srfi-34)
   #:use-module (srfi srfi-37)
   #:autoload   (distro) (find-packages-by-name)
   #:export (guix-build))
@@ -169,6 +170,16 @@ Report bugs to: ~a.~%") "@PACKAGE_BUGREPORT@"))
   (setvbuf (current-output-port) _IOLBF)
   (setvbuf (current-error-port) _IOLBF)
 
+  (guard (c ((package-input-error? c)
+             (let* ((package  (package-error-package c))
+                    (input    (package-error-invalid-input c))
+                    (location (package-location package))
+                    (file     (location-file location))
+                    (line     (location-line location))
+                    (column   (location-column location)))
+               (leave (_ "~a:~a:~a: error: package `~a' has an invalid input: ~s~%")
+                      file line column
+                      (package-full-name package) input))))
     (let* ((opts (parse-options))
            (src? (assoc-ref opts 'source?))
            (sys  (assoc-ref opts 'system))
@@ -230,4 +241,8 @@ Report bugs to: ~a.~%") "@PACKAGE_BUGREPORT@"))
                                                 (derivation-path->output-path
                                                  d out-name)))
                                               (derivation-outputs drv)))))
-                           drv))))))
+                             drv)))))))
+
+;; Local Variables:
+;; eval: (put 'guard 'scheme-indent-function 1)
+;; End:

guix/packages.scm

@@ -61,8 +61,10 @@
             package-cross-derivation
 
             &package-error
+            package-error?
             package-error-package
             &package-input-error
+            package-input-error?
             package-error-invalid-input))
 
 ;;; Commentary: