bug#27100: And another unnecessary backtrace by guix/build/download.scm

[Danny Milosavljevic <dannym@scratchpost.org>]

Backtrace:
          12 (primitive-load "/x/home/dannym/src/guix/scripts/guix")
In guix/ui.scm:
   1264:8 11 (run-guix-command _ . _)
In guix/scripts/import.scm:
   114:11 10 (guix-import . _)
In guix/scripts/import/pypi.scm:
    84:19  9 (guix-import-pypi . _)
In guix/import/pypi.scm:
   279:17  8 (pypi->guix-package _)
In ice-9/boot-9.scm:
    837:9  7 (catch srfi-34 #<procedure 2ced7e0 at guix/import/json…> …)
In guix/import/json.scm:
    32:17  6 (_)
In guix/http-client.scm:
   239:25  5 (http-fetch _ #:port _ #:text? _ #:buffered? _ # _ # _ # …)
In guix/build/download.scm:
    520:4  4 (open-connection-for-uri _ #:timeout _ # _)
   391:34  3 (tls-wrap #<input-output: socket 16> "pypi.python.org" # …)
    308:4  2 (make-credendials-with-ca-trust-files _)
In srfi/srfi-1.scm:
    640:9  1 (for-each #<procedure 2cf0b60 at guix/build/download.s…> …)
In unknown file:
           0 (set-certificate-credentials-x509-trust-file! #<certif…> …)

ERROR: In procedure set-certificate-credentials-x509-trust-file!:
ERROR: Throw to key `gnutls-error' with args `(#<gnutls-error-enum Fehler beim Dateilesen.> set-certificate-credentials-x509-trust-file!)'.

Again without the actual interesting information (the file in question), which is presumably cut off by the "..." in the backtrace.  Btw how can I turn the cutoff off?