Improve Foreign Key error handling.

We don't know how to parse the PostgreSQL condition sent when there is a
fkey error... and the message would not contain the row number where that
error happened anyway.

At the moment it means that the retry-batch facility errors out for failing
to realize that NIL isn't a number on which we can do arithmetic, which in
itself in a little sad.

In this patch we install a condition handler that knows how to deal with
retry-batch failing, so that pgloader may try and continue rather than
appear locked to the user, when I suspect that the debugger is waiting for
input.

See #836, where that's the first half of the fix. The real fix is to handle
foreign key errors correctly of course.

src/pg-copy/copy-rows-in-batch.lisp

@@ -67,9 +67,13 @@
         (log-message :error "PostgreSQL [~s] ~a" table-name condition)
         ;; clean the current transaction before retrying new ones
         (let ((errors
-               (retry-batch table columns batch condition)))
+               (handler-case
+                   (retry-batch table columns batch condition)
+                 (condition (e)
+                   (log-message :error "BUG: failed to retry-batch: ~a" e)
+                   (batch-count batch)))))
           (log-message :debug "retry-batch found ~d errors" errors)
-          (update-stats :data table :rows (- errors))))
+          (update-stats :data table :errs errors :rows (- errors))))
 
       (postgresql-unavailable (condition)