Show prepare button if status switches to failed.

app/templates/corpora/corpus.html.j2

@@ -147,7 +147,7 @@
       statusElement.classList.add(CorpusList.STATUS_COLORS[status] || CorpusList.STATUS_COLORS['default']);
       statusElement.innerText = status;
       var analyseBtn = document.getElementById('analyse');
-      if (status === 'prepared' || status === 'analysing') {
+      if (status === 'prepared' || status === 'analysing' || status === 'failed') {
         analyseBtn.classList.remove('hide', 'disabled');
       } else if (status === 'start analysis' || status === 'stop analysis') {
         analyseBtn.classList.remove('hide');