[Reports] Improve error message when html generation fails.

Per Chris Lam's comment 7 on https://bugs.gnucash.org/show_bug.cgi?id=799148.

gnucash/report/gnc-report.cpp

@@ -230,10 +230,17 @@ gnc_run_report_with_error_handling (gint report_id, gchar ** data, gchar **errms
     }
     else
     {
-        *errmsg = scm_is_string (captured_error) ? gnc_scm_to_utf8_string (captured_error) :
+        constexpr const char* with_err = "Report %s failed to generate html: %s";
+        constexpr const char* without_err = "Report %s Failed to generate html but didn't raise a Scheme exception.";
+        auto scm_err = scm_is_string (captured_error) ? gnc_scm_to_utf8_string (captured_error) :
             g_strdup ("");
+
+        if (scm_err && *scm_err)
+            *errmsg = g_strdup_printf (with_err, gnc_report_name (report), scm_err);
+        else
+            *errmsg = g_strdup_printf (without_err, gnc_report_name (report));
+
         *data = nullptr;
-        PWARN ("Error in report: %s", *errmsg);
         return FALSE;
     }
 }