Move evaluation errors from evaluations to EvaluationErrors, a new table

DBIx likes to eagerly select all columns without a way to really tell
it so. Therefore, this splits this one large column in to its own
table.

I'd also like to make "jobsets" use this table too, but that is on hold
to stop the bleeding caused by the extreme amount of traffic this is
causing.

src/sql/hydra.sql

@@ -437,13 +437,17 @@ create table SystemTypes (
     maxConcurrent integer not null default 2
 );
 
+create table EvaluationErrors (
+    id            serial primary key not null,
+    errorMsg      text,    -- error output from the evaluator
+    errorTime     integer  -- timestamp associated with errorMsg
+);
 
 create table JobsetEvals (
     id            serial primary key not null,
     jobset_id     integer not null,
 
-    errorMsg      text, -- error output from the evaluator
-    errorTime     integer, -- timestamp associated with errorMsg
+    evaluationerror_id integer,
 
     timestamp     integer not null, -- when this entry was added
     checkoutTime  integer not null, -- how long obtaining the inputs took (in seconds)
@@ -471,7 +475,8 @@ create table JobsetEvals (
     nixExprInput  text, -- name of the jobsetInput containing the Nix or Guix expression
     nixExprPath   text, -- relative path of the Nix or Guix expression
 
-    foreign key   (jobset_id) references Jobsets(id) on delete cascade
+    foreign key   (jobset_id) references Jobsets(id) on delete cascade,
+    foreign key   (evaluationerror_id) references EvaluationErrors(id) on delete set null
 );