_artifactcache/artifactcache.py: Rephrase failure message

It was saying "There is not enough space to build the given element.",
this makes me think the error is associated to a specific element, but
this does not make sense to show up in a cleanup task.

Instead say "There is not enough space to complete the build.", which
should be more clear that even after cleaning up there is not enough
space.

buildstream/_artifactcache/artifactcache.py

@@ -249,7 +249,7 @@ class ArtifactCache():
                 # FIXME: Asking the user what to do may be neater
                 default_conf = os.path.join(os.environ['XDG_CONFIG_HOME'],
                                             'buildstream.conf')
-                detail = ("There is not enough space to build the given element.\n"
+                detail = ("There is not enough space to complete the build.\n"
                           "Please increase the cache-quota in {}."
                           .format(self.context.config_origin or default_conf))