sign-pull: improve error handling

Use glnx_* functions in signature related pull code for clear
error handling.

Signed-off-by: Denis Pynkin <denis.pynkin@collabora.com>

tests/test-signed-pull-summary.sh

@@ -155,7 +155,7 @@ do
     if ${OSTREE} --repo=repo pull origin main 2>err.txt; then
         assert_not_reached "Successful pull with invalid ${engine} signature"
     fi
-    assert_file_has_content err.txt "Can't verify summary"
+    assert_file_has_content err.txt "signed with unknown key"
     mv ${test_tmpdir}/ostree-srv/gnomerepo/summary.sig{.good,}
     echo "ok ${engine} pull with invalid ${engine} summary signature fails"
 
@@ -223,7 +223,7 @@ cp ${test_tmpdir}/ostree-srv/gnomerepo/summary.sig{.2,}
 if ${OSTREE} --repo=repo pull origin main 2>err.txt; then
     assert_not_reached "Successful pull with old summary"
 fi
-assert_file_has_content err.txt "Can't verify summary"
+assert_file_has_content err.txt "signed with unknown key"
 assert_has_file repo/tmp/cache/summaries/origin
 assert_has_file repo/tmp/cache/summaries/origin.sig
 cmp repo/tmp/cache/summaries/origin ${test_tmpdir}/ostree-srv/gnomerepo/summary.1 >&2