context: add a PROGRESS: header in $BEESSTATUS

Make it clearer where the progress information goes.

Also add placeholder text so the progress section isn't empty at startup,
when the progress hasn't been calculated yet.

Signed-off-by: Zygo Blaxell <bees@furryterror.org>

src/bees-context.cc

@@ -98,6 +98,7 @@ BeesContext::dump_status()
 		TaskMaster::print_queue(ofs);
 #endif
 
+		ofs << "PROGRESS:\n";
 		ofs << get_progress();
 
 		ofs.close();
@@ -125,6 +126,9 @@ string
 BeesContext::get_progress()
 {
 	unique_lock<mutex> lock(m_progress_mtx);
+	if (m_progress_str.empty()) {
+		return "[No progess estimate available]\n";
+	}
 	return m_progress_str;
 }