Catch FileserverConfigError exceptions on master startup

This causes the master to fail to start when there are fileserver config
issues, leading people quicker to the log to investigate, where they
will find a meaningful error message. This should it much quicker and
easier for users to realize there are gitfs/hgfs/svnfs fileserver config
problems.

Fixes saltstack#21021.

salt/master.py

@@ -358,6 +358,13 @@ def _pre_flight(self):
                 'Failed to load fileserver backends, the configured backends '
                 'are: {0}'.format(', '.join(self.opts['fileserver_backend']))
             )
+        else:
+            # Run init() for all backends which support the function, to
+            # double-check configuration
+            try:
+                fileserver.init()
+            except FileserverConfigError as exc:
+                errors.append('{0}'.format(exc))
         if not self.opts['fileserver_backend']:
             errors.append('No fileserver backends are configured')
         if errors: