fix: handle curl error on delete branch

backport.sh

@@ -144,11 +144,12 @@ delete_branch() {
 
   local status
   status=$(curl -XDELETE -fsSL \
+    --fail \
     --output /dev/null \
     -w '%{http_code}' \
     -H 'Accept: application/vnd.github.v3+json' \
     -H "Authorization: Bearer ${INPUT_TOKEN}" \
-    "$refs_url/heads/$branch")
+    "$refs_url/heads/$branch" || true)
 
   if [[ "${status}" == 204 || "${status}" == 422 ]]; then
     return 0

spec/backport_spec.sh

@@ -230,7 +230,7 @@ EOF
 
       It 'Deletes branches'
         When call delete_branch backport/123-to-branch
-        The value "$(cat "${curl_args}")" should equal "-XDELETE -fsSL --output /dev/null -w %{http_code} -H Accept: application/vnd.github.v3+json -H Authorization: Bearer ${INPUT_TOKEN} git-refs-url/heads/backport/123-to-branch"
+        The value "$(cat "${curl_args}")" should equal "-XDELETE -fsSL --fail --output /dev/null -w %{http_code} -H Accept: application/vnd.github.v3+json -H Authorization: Bearer ${INPUT_TOKEN} git-refs-url/heads/backport/123-to-branch"
       End
     End
 
@@ -246,7 +246,7 @@ EOF
 
       It 'Doesn''t fail on deleted branches'
         When call delete_branch backport/123-to-branch
-        The value "$(cat "${curl_args}")" should equal "-XDELETE -fsSL --output /dev/null -w %{http_code} -H Accept: application/vnd.github.v3+json -H Authorization: Bearer ${INPUT_TOKEN} git-refs-url/heads/backport/123-to-branch"
+        The value "$(cat "${curl_args}")" should equal "-XDELETE -fsSL --fail --output /dev/null -w %{http_code} -H Accept: application/vnd.github.v3+json -H Authorization: Bearer ${INPUT_TOKEN} git-refs-url/heads/backport/123-to-branch"
       End
     End