Tell the user the filesize-limit if the screenshot is too big.

src/cgi.cpp

@@ -71,7 +71,8 @@ entry_type parse_formdata()
         constexpr size_t size_limit{1024 * 1024 * 2}; // 2 MiB.
         if (screenshot->getDataLength() > size_limit)
         {
-            throw runtime_error{"Filesize too big"};
+            throw runtime_error{"The screenshot is too big. "
+                                "The limit is 2 MiB."};
         }
 
         const string filepath{files::get_tmpdir() / screenshot->getFilename()};