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/sql/hydra.sql

@@ -192,6 +192,7 @@ create table Builds (
     --   8 = cached failure [steps only; builds use isCachedBuild]
     --   9 = unsupported system type
     --  10 = log limit exceeded
+    --  11 = NAR size limit exceeded
     buildStatus   integer,
 
     size          bigint,