Remove the errorMsg column from the Builds table

The queue runner no longer uses this field, and it doesn't provide
very interesting historical data (mostly SSH failures), but it takes
up a lot of space. Also, it contained some bad UTF-8 which was
preventing an upgrade to Postgres 9.5, so a good occasion to get rid
of it.

src/sql/hydra.sql

@@ -184,7 +184,7 @@ create table Builds (
     --   0 = succeeded
     --   1 = build of this derivation failed
     --   2 = build of some dependency failed
-    --   3 = other failure (see errorMsg)
+    --   3 = other failure
     --   4 = build cancelled (removed from queue; never built)
     --   5 = build not done because a dependency failed previously (obsolete)
     --   6 = failure with output
@@ -193,8 +193,6 @@ create table Builds (
     --  10 = log limit exceeded
     buildStatus   integer,
 
-    errorMsg      text, -- error message in case of a Nix failure
-
     size          bigint,
     closureSize   bigint,