Handle non-zero exit status from psql more gracefully

* lisp/progmodes/sql.el (sql-postgres-list-databases): Handle non-zero
exit statuses from `psql -ltX' more gracefully by returning nil.

* test/lisp/progmodes/sql-tests.el
(sql-tests-postgres-list-databases-error): New test.

lisp/progmodes/sql.el

@@ -1095,9 +1095,10 @@ add your name with a \"-U\" prefix (such as \"-Umark\") to the list."
   "Return a list of available PostgreSQL databases."
   (when (executable-find sql-postgres-program)
     (let ((res '()))
-      (dolist (row (process-lines sql-postgres-program "-ltX"))
-        (when (string-match "^ \\([[:alnum:]-_]+\\) +|.*" row)
-          (push (match-string 1 row) res)))
+      (ignore-errors
+        (dolist (row (process-lines sql-postgres-program "-ltX"))
+          (when (string-match "^ \\([[:alnum:]-_]+\\) +|.*" row)
+            (push (match-string 1 row) res))))
       (nreverse res))))
 
 ;; Customization for Interbase

test/lisp/progmodes/sql-tests.el

@@ -43,5 +43,15 @@
     (should (equal (sql-postgres-list-databases)
                    '("db-name-1" "db_name_2")))))
 
+(ert-deftest sql-tests-postgres-list-databases-error ()
+  "Test that nil is returned when `psql -ltX' fails."
+  (cl-letf
+      (((symbol-function 'executable-find)
+        (lambda (_command) t))
+       ((symbol-function 'process-lines)
+        (lambda (_program &rest _args)
+          (error))))
+    (should-not (sql-postgres-list-databases))))
+
 (provide 'sql-tests)
 ;;; sql-tests.el ends here