build-system/gnu: Report invocation errors in a human-friendly way.

* guix/build/utils.scm (report-invoke-error): New procedure.
* guix/build/gnu-build-system.scm (gnu-build): Guard against
'invoke-error?'.

guix/build/gnu-build-system.scm

@@ -790,6 +790,9 @@ in order.  Return #t if all the PHASES succeeded, #f otherwise."
   ;; Encoding/decoding errors shouldn't be silent.
   (fluid-set! %default-port-conversion-strategy 'error)
 
+  (guard (c ((invoke-error? c)
+             (report-invoke-error c)
+             (exit 1)))
     ;; The trick is to #:allow-other-keys everywhere, so that each procedure in
     ;; PHASES can pick the keyword arguments it's interested in.
     (every (match-lambda
@@ -814,4 +817,4 @@ in order.  Return #t if all the PHASES succeeded, #f otherwise."
                   ;; Dump the environment variables as a shell script, for handy debugging.
                   (system "export > $NIX_BUILD_TOP/environment-variables")
                   result))))
-         phases))
+           phases)))

guix/build/utils.scm

@@ -98,6 +98,7 @@
             invoke-error-exit-status
             invoke-error-term-signal
             invoke-error-stop-signal
+            report-invoke-error
 
             locale-category->string))
 
@@ -622,6 +623,11 @@ Where every <*-phase-name> is an expression evaluating to a symbol, and
     ((_ phases (add-after old-phase-name new-phase-name new-phase))
      (alist-cons-after old-phase-name new-phase-name new-phase phases))))
 
+
+;;;
+;;; Program invocation.
+;;;
+
 (define-condition-type &invoke-error &error
   invoke-error?
   (program      invoke-error-program)
@@ -643,6 +649,17 @@ if the exit code is non-zero; otherwise return #t."
                          (stop-signal (status:stop-sig code))))))
     #t))
 
+(define* (report-invoke-error c #:optional (port (current-error-port)))
+  "Report to PORT about C, an '&invoke-error' condition, in a human-friendly
+way."
+  (format port "command~{ ~s~} failed with ~:[signal~;status~] ~a~%"
+          (cons (invoke-error-program c)
+                (invoke-error-arguments c))
+          (invoke-error-exit-status c)
+          (or (invoke-error-exit-status c)
+              (invoke-error-term-signal c)
+              (invoke-error-stop-signal c))))
+
 
 ;;;
 ;;; Text substitution (aka. sed).