web: Check for the existance of SSL related files.

This adds back the previous behaviour of the nginx-service-type, where the
service would check at the time when the configuration is generated if the SSL
certificate and certificate key file exists.

* gnu/services/web.scm (emit-nginx-server-config): Add back check for SSL
  related files.

gnu/services/web.scm

@@ -157,6 +157,16 @@ of index files."
             (syntax-parameterize ((<> (identifier-syntax x*)))
               (list tail ...))
             '())))
+    (for-each
+     (match-lambda
+      ((record-key . file)
+       (if (and file (not (file-exists? file)))
+           (error
+            (simple-format
+             #f
+             "~A in the nginx configuration for the server with name \"~A\" does not exist" record-key server-name)))))
+     `(("ssl-certificate"     . ,ssl-certificate)
+       ("ssl-certificate-key" . ,ssl-certificate-key)))
     (list
      "    server {\n"
      (and/l http-port  "      listen " (number->string <>) ";\n")