[bugfix] Avoid 400 error when pkg not found

Previously, a request for a package like packages/foo.html or packages/monex.html?eXist-db-min-version=1.0.0 would return XQuery errors. This provides a proper 404 error and a human-readable error message for each condition.

modules/app.xql

@@ -49,7 +49,16 @@ declare function app:view-package($node as node(), $model as map(*), $mode as xs
  let $package := $app-versions[@path eq $compatible-xar]
  let $show-details := true()
  return
- app:package-to-list-item($package, $show-details)
+ if (exists($package)) then
+ app:package-to-list-item($package, $show-details)
+ else
+ (
+ response:set-status-code(404),
+ if (exists($app)) then
+ <li class="package text-warning">Package {$abbrev} requires a newer version of eXist.</li>
+ else
+ <li class="package text-warning">No package {$abbrev} is available.</li>
+ )
 };
 
 declare function app:package-to-list-item($app as element(app), $show-details as xs:boolean) {