download: raise a more obvious errors if no mirrors can be found

Signed-off-by: Mattia Rizzolo <mapreri@ubuntu.com>

uitwww/download.py

@@ -66,12 +66,18 @@ class Downloads:
             if self._cache is not None:
                 self._mirrors = self._cache["mirrors"]
             else:
-                self._mirrors = {
-                    distro: list(sorted(launchpad.get_cdimage_mirrors(
+                self._mirrors = {}
+                for distro in self.config["mirrors"]["for"]:
+                    found_mirrors = list(sorted(launchpad.get_cdimage_mirrors(
                          distro, self.config["mirrors"]["country"]
-                    ))) for distro in self.config["mirrors"]["for"]
-                }
-
+                    )))
+                    if found_mirrors:
+                        self._mirrors[distro] = found_mirrors
+                if not self._mirrors:
+                    raise ValueError(
+                        "No available mirrors found for %(for)s in %(country)s"
+                        % self.config["mirrors"]
+                    )
         return self._mirrors
 
     @property