scripts: substitute: Tweak error reporting in process-substitution.

The call-with-connection-error-handling was added in
20c08a8a45, but that error handling was
previously inside of open-connection-for-uri/maybe, which is related
to (call-)with-cached-connection which was used in process-substitution, but
only actually used with call-with-cached-connection when used in
fetch-narinfos.

There's some handling for similar errors within with-networking, which is used
within process-substitution.

* guix/scripts/substitute.scm (process-substitution): Remove
call-with-connection-error-handling call.

guix/scripts/substitute.scm

@@ -448,14 +448,11 @@ (define (fetch uri)
              (warning (G_ "while fetching ~a: server is somewhat slow~%")
                       (uri->string uri))
              (warning (G_ "try `--no-substitutes' if the problem persists~%")))
-           (call-with-connection-error-handling
-            uri
-            (lambda ()
-              (with-cached-connection uri port
-                (http-fetch uri #:text? #f
-                            #:port port
-                            #:keep-alive? #t
-                            #:buffered? #f)))))))
+           (with-cached-connection uri port
+             (http-fetch uri #:text? #f
+                         #:port port
+                         #:keep-alive? #t
+                         #:buffered? #f)))))
       (else
        (leave (G_ "unsupported substitute URI scheme: ~a~%")
               (uri->string uri)))))