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/root/build.tt

@@ -280,11 +280,6 @@ FOR step IN steps; IF step.busy; busy = 1; END; END;
         </table>
       [% END %]
 
-      [% IF build.errormsg && build.buildstatus != 5 %]
-        <h2 id="nix-error">Nix error output</h2>
-        <pre class="buildlog">[% HTML.escape(build.errormsg) %]</pre>
-      [% END %]
-
     [% END %]
 
   </div>