Log and exit on metadata update errors at startup

A bad system repo can cause lorax-composer to fail to start. Instead of
a traceback log the error and exit.

(note that the exit still results in an OSError traceback due to part of
it running as root, this needs to be addressed in another commit).

(cherry picked from commit b1dd22afa6)

src/pylorax/api/dnfbase.py

@@ -82,7 +82,11 @@ def get_base_object(conf):
 
     # Update the metadata from the enabled repos to speed up later operations
     log.info("Updating repository metadata")
+    try:
         dbo.fill_sack(load_system_repo=False)
         dbo.read_comps()
+    except dnf.exceptions.Error as e:
+        log.error("Failed to update metadata: %s", str(e))
+        raise RuntimeError("Fetching metadata failed: %s" % str(e))
 
     return dbo

src/sbin/lorax-composer

@@ -245,7 +245,11 @@ if __name__ == '__main__':
     make_dnf_dirs(server.config["COMPOSER_CFG"])
 
     # Get a dnf.Base to share with the requests
+    try:
         dbo = get_base_object(server.config["COMPOSER_CFG"])
+    except RuntimeError:
+        # Error has already been logged. Just exit cleanly.
+        sys.exit(1)
     server.config["DNFLOCK"] = DNFLock(dbo=dbo, lock=Lock())
 
     # Depsolve the templates and make a note of the failures for /api/status to report