Check whether a modulemd file exists before download/parse

There's a modulemd.arch.txt for each archicture the module has been
built for.
If we're, for example, trying to get module info for aarch64
and the modulemd.aarch64.txt doesn't exist, it most likely means
that there's been no aarch64 build.

mtps-get-module

@@ -21,6 +21,7 @@ if [ -z "${YUMDNFCMD:-}" ]; then source "$(dirname "${BASH_SOURCE[0]}")/mtps-set
 
 RET_NO_RPMS_IN_BREW=7
 RET_NO_RPMS_IN_REPOS=8
+RET_NO_MODULE_ARCH=9
 
 builds_from_answer() {
     local xml="$1" && shift
@@ -174,6 +175,17 @@ mk_url_mmd() {
     echo "$url"
 }
 
+url_exists() {
+    local url="$1" && shift
+    response=$(curl --silent --head "$url" | grep HTTP)
+    if [[ $response != *" 404 "* && $response != *" 403 "* ]]; then
+        return 0
+    else
+        echo "$url does not exist"
+        return 1
+    fi
+}
+
 download_artifact() {
     local url="$1" && shift
     local dstdir="$1" && shift
@@ -453,6 +465,10 @@ if [[ -z "$builds_in_m" ]]; then
 fi
 
 mmd_url="$(mk_url_mmd "$m_nvr" "$ARCH")"
+if ! url_exists "$mmd_url"; then
+  echo "$m_nvr has probably not been built for $ARCH"
+  exit $RET_NO_MODULE_ARCH
+fi
 mmd_file="$(basename "$mmd_url")"
 mmd_dir="$(mktemp --directory)"
 download_artifact "$mmd_url" "$mmd_dir"