Give an error when no text could be found inside a URL.

copyvios/checker.py

@@ -41,7 +41,11 @@ def _get_results(query):
         if urlparse(query.url).scheme not in ["http", "https"]:
             query.error = "bad URI"
             return
-        query.result = page.copyvio_compare(query.url)
+        result = page.copyvio_compare(query.url)
+        if result.source_chain is page.EMPTY:
+            query.error = "no data"
+            return
+        query.result = result
         query.result.cached = False
     else:
         conn = get_cache_db()

templates/index.mako

@@ -7,6 +7,10 @@
         <div id="info-box" class="red-box">
             <p>Unsupported URI scheme: <a href="${query.url | h}">${query.url | h}</a>.</p>
         </div>
+    % elif query.error == "no data":
+        <div id="info-box" class="red-box">
+            <p>Couldn't find any text in <a href="${query.url | h}">${query.url | h}</a>. <i>Note:</i> only HTML and plain text pages are supported, and content generated by JavaScript or found inside iframes is ignored.</p>
+        </div>
     % elif not query.site:
         <div id="info-box" class="red-box">
             <p>The given site (project=<b><span class="mono">${query.project | h}</span></b>, language=<b><span class="mono">${query.lang | h}</span></b>) doesn't seem to exist. It may also be closed or private. <a href="//${query.lang | h}.${query.project | h}.org/">Confirm its URL.</a></p>