[core] Improve error message when GNU tar is missing

This has led to numerous reported issues in the past. Spacemacs can work without
GNU tar, because quelpa installation only fails for some packages; so it should
suffice to add the message in that case.

Also, remove the superfluous quote from the condition name error.

Author: fnussbaum

core/core-configuration-layer.el

@@ -1717,9 +1717,14 @@ RNAME is the name symbol of another existing layer."
             (oset pkg lazy-install nil))
            (t (configuration-layer//warning "Cannot install package %S."
                                             pkg-name)))
-        ('error
+        (error
          (configuration-layer//error
-          (concat "\nAn error occurred while installing %s " "(error: %s)\n")
+          (concat "An error occurred while installing %s " "(error: %s)"
+                  (when (and (equal err '(wrong-type-argument package-desc nil))
+                             (boundp 'quelpa--tar-type)
+                             (eq quelpa--tar-type 'bsd))
+                    ". It looks like GNU tar is not installed, or `quelpa-build-tar-executable' \
+needs to be customized to a GNU tar executable."))
           pkg-name
           err)
          (spacemacs//redisplay))))))