slapgrid: do NOT raise in case of servererror when calling /softwareAvailable.

Mostly informational, it is more improtant to have a functional slapgrid.
Also: remove trailing spaces.

slapos/grid/slapgrid.py

@@ -468,7 +468,7 @@ class Slapgrid(object):
         if state == 'available':
           try:
             software_release.available()
-          except NotFoundError:
+          except (NotFoundError, ServerError):
             pass
         elif state == 'destroyed':
           try: