packages: Set the port conversion strategy to 'error'.

Suggested by Mark H Weaver.

* guix/build/gnu-build-system.scm (gnu-build): Set
  %DEFAULT-PORT-CONVERSION-STRATEGY to 'error.
* guix/packages.scm (patch-and-repack)[builder]: Likewise.

guix/build/gnu-build-system.scm

@@ -499,6 +499,9 @@ in order.  Return #t if all the PHASES succeeded, #f otherwise."
   (setvbuf (current-output-port) _IOLBF)
   (setvbuf (current-error-port) _IOLBF)
 
+  ;; Encoding/decoding errors shouldn't be silent.
+  (fluid-set! %default-port-conversion-strategy 'error)
+
   ;; 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

guix/packages.scm

@@ -412,6 +412,9 @@ IMPORTED-MODULES specify modules to use/import for use by SNIPPET."
                     (srfi srfi-1)
                     (guix build utils))
 
+       ;; Encoding/decoding errors shouldn't be silent.
+       (fluid-set! %default-port-conversion-strategy 'error)
+
        (let ((locales (assoc-ref %build-inputs "locales"))
              (out     (assoc-ref %outputs "out"))
              (xz      (assoc-ref %build-inputs "xz"))