Make the output size limit configurable

The maximum output size per build step (as the sum of the NARs of each
output) can be set via hydra.conf, e.g.

  max-output-size = 1000000000

The default is 2 GiB.

Also refactored the build error / status handling a bit.

src/root/build.tt

@@ -65,6 +65,8 @@ FOR step IN steps; IF step.busy; busy = 1; END; END;
                 <span class="error">Unsupported system type</span>
               [% ELSIF step.status == 10 %]
                 <span class="error">Log limit exceeded</span>
+              [% ELSIF step.status == 11 %]
+                <span class="error">Output limit exceeded</span>
               [% ELSIF step.errormsg %]
                 <span class="error">Failed: [% HTML.escape(step.errormsg) %]</span>
               [% ELSE %]