Improve select_file error handling

When tar/zip are unhappy with their inputs, they will yield no list items,
and we want to treat that as fatal.

When the list is empty or the grep fails to find something, we want to explain
which case happened before dying.

Author: jsoref

gh-program-downloader

@@ -134,13 +134,27 @@ download_artifact() {
 }
 
 select_file() {
-  (
-    if [ -n "$file_re" ]; then
-      grep -E "$file_re"
+  list=$(mktemp)
+  cat - > "$list"
+  if [ ! -s "$list" ]; then
+    echo "No files in archive" >&2
+    false
+  fi
+  if [ -n "$file_re" ]; then
+    result=$(
+      grep -E "$file_re" "$list" | tail -1
+    )
+    rm -f "$list"
+    if [ -n "$result" ]; then
+      echo "$result"
     else
-      cat -
+      echo "file_re ($file_re) did not match any files in archive" >&2
+      false
     fi
-  ) | tail -1
+  else
+    tail -1 "$list"
+    rm -f "$list"
+  fi
 }
 
 strip_zip_file_size_date_and_time_from_directory_listing() {